Research Article

Is “Huh?” a Universal Word? Conversational Infrastructure and the Convergent Evolution of Linguistic Items

  • Mark Dingemanse mail,

    Affiliation: Language and Cognition Department, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands

  • Francisco Torreira,

    Affiliation: Language and Cognition Department, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands

  • N. J. Enfield

    Affiliations: Language and Cognition Department, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands, Centre for Language Studies, Radboud University, Nijmegen, The Netherlands

  • Published: November 08, 2013
  • DOI: 10.1371/journal.pone.0078273

2 Apr 2014: The PLOS ONE Staff (2014) Correction: Is "Huh?" a Universal Word? Conversational Infrastructure and the Convergent Evolution of Linguistic Items. PLoS ONE 9(4): e94620. doi: 10.1371/journal.pone.0094620 | View correction


A word like Huh?–used as a repair initiator when, for example, one has not clearly heard what someone just said– is found in roughly the same form and function in spoken languages across the globe. We investigate it in naturally occurring conversations in ten languages and present evidence and arguments for two distinct claims: that Huh? is universal, and that it is a word. In support of the first, we show that the similarities in form and function of this interjection across languages are much greater than expected by chance. In support of the second claim we show that it is a lexical, conventionalised form that has to be learnt, unlike grunts or emotional cries. We discuss possible reasons for the cross-linguistic similarity and propose an account in terms of convergent evolution. Huh? is a universal word not because it is innate but because it is shaped by selective pressures in an interactional environment that all languages share: that of other-initiated repair. Our proposal enhances evolutionary models of language change by suggesting that conversational infrastructure can drive the convergent cultural evolution of linguistic items.


A fundamental tenet of linguistic science is that the sound of a word has a purely arbitrary connection to the word's meaning [1], [2]. Thus, the sound of the word dog in English is connected to the concept ‘dog’ by historical accident and not by any natural connection; roughly the same concept is just as well denoted in French by chien, in German by hund, and in Japanese by inu. But it is not that a word can have just any vocal sound. While the possibility space for sound systems of the world's language is enormous, any given language makes use of only a restricted portion of the possible sounds [3], [4]. It follows from these two basic principles –the ‘arbitrariness of the sign’, and the ‘selectiveness of particular sound systems’– that the words that exist in the world's languages should sound quite different from each other, and that the likelihood that there are universal words is extremely small. But in this study we present a striking exception to this otherwise robust rule. From a systematic comparison of 10 spoken languages from 5 continents we find evidence suggesting that a word like ‘Huh?’–used as a ‘repair initiator’ when, for example, one has not clearly heard what someone just said [5], [6]–is a universal word.

There are two distinct claims being made here: 1. that Huh? is universal, and 2. that Huh? is a word. In support of the first claim, we show that the similarities in form and function of an interjection with the specific function of repair initiation are very much greater across languages than chance coincidence would admit. In fact the variation in form in unrelated languages across the globe is about the same as the variation we find in the way any regular word (e.g., dog) is pronounced across dialects of English. In support of the second claim, we show that Huh? meets the criteria of a word in the sense of being a conventional lexical sign which must be learnt. Thus, in contrast to what has been presumed for interjections in general [7], [8] and for huh? in particular [9], [10], we find that this item is linguistic in nature rather than being a mere grunt or non-lexical sound. We show that the form is locally calibrated in ways that show it fitting within different language systems. Huh? may be a non-prototypical word, but it is a word.

Finally, we address the question of why all languages should have such a word and why its form should be so similar across languages. We observe that this item fulfils a crucial need shared by all languages –the efficient signalling of problems of hearing and understanding– and we propose that its form is constrained by selective pressures in a conversational environment that is essentially the same in all languages. Consider a case from English [10]:

Extract 1 American English [NB, 1:1:19]

After speaker G makes a statement, speaker E utters the interjection huh?. This is followed by a repetition of the original statement by G. The technical term for this type of sequence is “open other-initiated repair”: repair is initiated not by the speaker of the first turn but by the other participant (“other-initiated”), and the repair initiator signals that there is a problem, but it leaves open what the problem is (“open”) [11]. The actual repair operation in response to this interjection is usually simply repetition, sometimes with slight modification. Extracts 2 and 3 show structurally identical sequences in two other languages: Siwu, a Kwa language spoken in Ghana, and Lao, a Tai-Kadai language spoken in Laos.

Extract 2 Siwu (Ghana) [Maize1_1017013]

Extract 3 Lao (Laos) [CONV_050815c_03.10]

These examples show that it is possible to identify the same conversational structure in unrelated languages. Essentially, this method gives us a natural control over conversational data, making possible systematic comparison across languages [12], [13]. Sequences of other-initiated repair have been identified in every spoken language investigated so far [14], [15], and as the examples show, the interjection in the pivotal turn can be remarkably similar. This leads to the question driving our study: is huh? in this context a universal word?

By compiling data from published literature we found that in thirty-one languages around the world, the interjection for other-initiated repair appears to be strongly similar (Figure 1). However, written sources are rarely explicit about the precise form, meaning, and use of interjections. The most reliable way to study a conversational interjection is by examining cases of actual use. Therefore we collected data from recordings of naturally occurring informal conversations in a sample of 10 languages from 5 continents, varying fundamentally in terms of phonology, word structure, and grammar (languages 1–10 in Figure 1). For optimal comparability, we studied the exact same conversational environment across languages: that of other-initiated repair (OIR), in which one participant produces a turn at talk, the other then signals some trouble with this turn, and finally the first produces a next turn which aims to solve the trouble, usually by means of repetition and/or modification. In some languages the interjection, or an item similar to it, was also found in other sequential environments, for instance to mark surprise or to pursue a response. Such alternative (and probably derived) uses provide insight in possible paths of semantic change, but we exclude them here to make sure we are comparing like with like.


Figure 1. Interjections for other-initiation of repair in thirty-one languages.

A word like huh? –used to initiate repair when, for example, one has not clearly heard what someone just said– is found in roughly the same form in spoken languages across the globe. Languages 1–10 are examined in detail in the present study, 11–20 from [14], 21–31 from sources cited. Locations are approximate. 1. Cha'palaa 2. Icelandic ha 3. Spanish e↗ 4. Siwu ã:↗ 5. Dutch h↗ 6. Italian ε:↗ 7. Russian a:↗ 8. Lao hã:↗ 9. Mandarin Chinese ã:↗ 10. Murrinh-Patha a:↗ 11. ‡Âkhoe Hai//om hε↗ 12. Chintang hã↗ 13. Duna 14. English hã↗ 15. French 16. Hungarian hm↗/ha↗ 17. Kri ha:↗ 18. Tzeltal hai↗ 19. Yélî Dnye 20. Yurakaré æ↗ 21. Lahu hãi [38] 22. Tai/Lue hy ˘↗/há↗ [92] 23. Japanese e↗ [93] 24. Korean e↗ [94] 25. German h [95] 26. Norwegian h[96] 27. Herero e↗ [97] 28. Kikongo e↗ [98] 29. Tzotzil e↗ [99] 30. Bequia Creole ha:↗ [100] 31. Zapotec aj↗ [101].


Earlier we found that all 10 languages in the sample make available two types of expressions to initiate repair in this conversational environment [14]: an interjection (comparable to English “huh?”) and a question word-based expression (comparable to English “what?”) – with the interjection being a dedicated, default form for open other-initiated repair, and the question word being recruited from a larger grammatical paradigm of question words. The question words for initiating repair in the languages in our sample are very different in phonetic form, with varying numbers of syllables, a wide range of different consonants and vowels, and many different combinations of speech sounds (Table 1). This is just as expected in a diverse language sample given the principle of the arbitrariness of the sign. Compared to the question words, the interjections for initiating repair are strikingly similar in form (Table 1). It is this exceptional similarity that we investigated in this study.


Table 1. Question words (“what?”) and interjections (“huh?”) for initiating repair in ten languages.


Materials and Methods

We collected 196 instances of the interjection for other-initiated repair (henceforth OIR interjection) in videotaped recordings of conversation in a worldwide sample of 10 languages (mean instances per language = 19.6, sd = 7.5). We used field recordings of maximally informal conversation because most written sources do not offer enough phonetic detail and people's intuitions about their behaviour can be different from their actual behaviour [16]. We examined at least ten tokens per language to find out whether or not the same articulatory target is aimed for within and across languages. All data were collected in accordance with protocols approved by the ethical review board of the Seventh EU Framework (240853 HSSLU). Informed consent was obtained from all participants according to standard practices [17], [18]. The data were anonymised and unlinked and there is no possibility of identification.

We used a two-stage approach to comparative analysis of the tokens. In an auditory analysis, we collected phonetic auditory judgements of interjection tokens by three annotators and combined them into graded measures along five phonetic dimensions for every single token (see File S1). In an instrumental analysis, we took acoustic measurements on a subset of tokens and used these to verify the auditory judgements for selected dimensions. The combination of auditory and instrumental approaches enabled us to carry out an analysis that is ecologically valid and well controlled.

In the auditory analysis, all interjection tokens (n = 196) were presented one by one in random order to three annotators independently. No separate information about language or recording was provided. Annotators listened to the audio clips with spectrograms and pitch tracks available on screen, and coded every token for five phonetic dimensions selected on the basis of preliminary observations of the range of variation: closure, nasality, vowel quality, intonation, and onset (see SI). Articulatory gestures in spoken language are essentially gradient [19]. Therefore, the coding results were combined into cumulative measures per token per dimension, allowing us to measure and display the variation in, for instance, vowel quality or consonant onset by language.

In the instrumental analysis, we took acoustic measurements of intonation and the first two vowel formants for languages in which token quantity and acoustic quality permitted this, namely Spanish and Cha'palaa. For Spanish, all tokens came from laboratory recordings of casual conversation [20]; for Cha'palaa, the large number of tokens in the field recordings permitted instrumental analysis. Some acoustically inferior interjection tokens (due to overlapping speech or ambient noise) and some tokens spoken by children were discarded. In total, 13 Cha'palaa tokens and 12 Spanish tokens were analysed instrumentally. Pitch values throughout each interjection were computed, and formant values of vowels were measured at the point of maximum intensity using the Burg method implemented in the software Praat [21].


All interjection tokens in all languages in our sample are syllable-like utterances consisting of one vowel-like sound optionally preceded by a consonant-like sound. We refer to these elements as syllable, vowel, and onset. We never found forms longer than one syllable and we never found any final consonant-like sounds.


Vowels can be characterized along three continuous articulatory dimensions: height (referring to the height of the tongue, associated with the F1 formant), backness (referring to the position of the tongue relative to the back of the mouth, associated with the F2 formant), and lip rounding. Within the two-dimensional space formed by height and backness, OIR interjections occupy only the low front central corner (Figure 2). Coding divided this corner of the space into four perceptual quadrants along two dimensions: Height (from low to mid) and Backness (from front to central). Within this restricted part of the space, most languages appear to aim for specific local targets (Figure 3). For instance, Cha'palaa tokens cluster in the low-central region, Spanish has a preference for the mid-front region, Italian clusters in the mid-central region, and Murrinh-Patha is mostly low. Some languages have a wider spread than others (e.g. Lao, Siwu). With respect to the third dimension, that of lip rounding, we found no variation: only unrounded vowels were found in all of the languages.


Figure 2. Average positions of the interjections in vowel space.

The vowel inventories of the world's languages tend to make maximal use of vowel space [41]. In contrast to this, the vowels of the OIR interjections all cluster in the same low-front region. Abbreviations: Cha'palaa (Cha), Dutch (Dut), Icelandic (Ice), Italian (Ita), Lao (Lao), Mandarin (Man), Murrinh-Patha (Mur), Russian (Rus), Siwu (Siw), Spanish (Spa).


Figure 3. Vowel quality of interjection tokens by language.

Although the vowel of the OIR interjections is limited to the low-front region, auditory analysis shows that within that region, not all languages target the same spot – the interjections appear to have distinct vowel targets.


To verify the validity of the auditory judgements, we performed acoustic measurements of the first two vowel formants (F1 and F2, associated with the height and backness dimensions) for Spanish and Cha'palaa (Figure 4). The two languages are statistically different on both of these dimensions (F1: B = −284.65, t = −9.34, p<.0001; F2: B = 398.54, t = 4.2, p<.0001). Spanish tokens have lower F1 and higher F2 values, consistent with the mid front vowel [e] found in the auditory judgments. The F1 of the Spanish interjection is in the same range as the F1 of the /e/ in a corpus of spontaneous Spanish [22], making it likely that the articulatory target of the interjection fits the phonology of the language. Cha'palaa tokens have higher F1 and lower F2 values, consistent with the low central vowel [a] found in the auditory judgements.


Figure 4. Formant values for the interjection vowels in Spanish (S) and Cha'palaa (C).

An instrumental analysis of interjection tokens from Spanish and Cha'palaa shows that the interjections have distinct, language-specific vowel targets, confirming the auditory analysis in Figure 3.



Within languages, the intonation of the interjection tokens is strongly consistent. In most languages in our sample it has rising pitch (Figure 5). Across many languages, rising pitch is associated with non-finality, uncertainty, and questioning [23], [24]. Exceptionally, in two languages in our sample the interjection has falling pitch: Icelandic and Cha'palaa. In these languages, falling intonation is the preferred intonation in wh-questions [14], [25], and the interjection shares its intonation with the question word-based expression for open repair initiation. The falling pitch of the OIR interjections in these languages thus appears to be calibrated to the local system of interrogative prosody. Across languages, then, the pitch of the interjections is best described as “questioning”, with the specific formal value determined by the local language system.


Figure 5. Intonation of the OIR interjection by language.

Intonation of the OIR interjection is rising in most languages and falling in some, but more accurately described as “questioning” in all. In this product plot [102], area of squares is proportional to token count: a larger square means more tokens.


To verify the validity of the auditory judgements we performed acoustic measurements on Spanish and Cha'palaa interjections, calculating pitch excursion as the difference between the pitch at the beginning and end of the contour. Pitch tracks in normalised time show rising contours for Spanish and falling contours for Cha'palaa, consistent with the auditory judgements (Figure 6). Spanish contours rise around 7 semitones on average, while Cha'palaa fall around 2 semitones. A model with pitch excursion as dependent response and language as predictor shows that the difference between the groups is statistically significant (B = 9.01, t = 8.04, p<.0001).


Figure 6. Pitch tracks for interjections in Spanish and Cha'palaa.

Instrumental analysis of pitch tracks in Spanish (n = 12) and Cha'palaa (n = 13) confirms the auditory analysis in Figure 5.



Most interjection tokens in most languages have no onset, but if there is one it is restricted to a glottal stop [ʔ] or a glottal fricative [h] (Figure 7). The direction in which tokens diverge from the no onset default appears to be influenced by the phonological system of the language, as follows: if an interjection token features an onset, that onset tends to approach one of the glottal consonants found in the phoneme inventory of the language.


Figure 7. Interjection onset by language.

Aspiration [h] and glottal stop [ʔ] onsets are at opposite ends of a continuum with no onset () in the middle. In most languages, ‘no onset’ is the default form, and the direction in which interjections diverge from this is related to the phonology of the language. Spanish is a special case because the laboratory recordings [20] allow the detection of even the slightest glottal constriction or aspiration. In this product plot [102], area of squares is proportional to token count: a larger square means more tokens.


Thus, Cha'palaa has a phonemic glottal stop/ʔ/[26], and many of its interjection tokens feature this sound. Dutch and Icelandic have phonemic/h/in onset position [27][29] and many of their interjection tokens feature this sound. Lao has both/ʔ/and/h/as distinctive sounds in onset position [30] and the onsets of its OIR interjection vary between /ʔ/,/ h/, and zero. The reverse holds true as well: if there are no glottal sounds in the phonology of the language, there is a high probability that the interjection will not feature a clear onset consonant. Thus, Mandarin, Murrinh-Patha and Russian have no phonemically contrastive glottal stop or fricative [31][33], and none (Mandarin & Russian) or very few (Murrinh-Patha) of their OIR interjection tokens feature these sounds. As Figure 7 also shows, the relation between the consonant inventory of the language and the onset of the interjection is not deterministic: presence of glottal consonants in the phonological inventory does not predict the occurrence of a consonant in all of its OIR interjections.


Table 2. Languages, field sites, and contributing researchers.


Nasality and mouth aperture

Some degree of nasality of the vowel was perceived in the majority of cases (Figure A in File S1). This may be connected to the fact that a slightly lowered velum is the neutral or resting position for the articulators [34]. It may also be linked to the glottal quality of the consonant onset; the affinity between nasality and glottality is well-known and has been explained on perceptual as well as articulatory grounds [35], [36].

Closed-mouth variants of the OIR interjection (e.g. m? n?) were observed in most of the languages in our sample, but the overall frequency was low and it was not the most common form in any of the languages (Figure B in File S1). Qualitative analysis suggests that closed-mouth forms are mainly used when speakers are close to each other. Since bilabial closure and a lowered velum are the neutral position of the speech articulators [34], these closed forms may be seen as an extreme articulatory reduction of the open-mouth form.


Is huh? a word?

In work on English conversations, the interjection huh? has been characterised as a “non-lexical token” [9] or a “non-lexical conversational sound” [8]. Yet our phonetic analysis shows that despite the overall similarity across languages, the OIR interjection is systematically calibrated to the language system in which it is integrated. This motivates the question whether huh? is a word. Two key characteristics of words are ‘integration’ – they are items in larger linguistic systems, and ‘conventionalisation’ – one cannot know them without having learnt them. Non-linguistic vocalisations like crying or grunting are the opposite on both counts: they are not integrated in linguistic systems, and one does not need to learn them to know them.


In all languages investigated, the sound of the OIR interjection shows some degree of calibration to local linguistic systems. Vowel targets are language-specific and appear to be drawn to existing phonemic targets, e.g. /e/ for Spanish and /a/ for Cha'palaa. Intonation melodies appear to be linked to the interrogative prosodic system, which may differ from language to language. The occurrence and quality of consonant onsets is related to the consonant inventory of the language. The interjection is also part of a larger paradigm of expressions for the other-initiation of repair, including, in English, other items like what? and pardon? [5], [15]. Huh? is thus an item integrated in several linguistic subsystems, from segmental and prosodic phonology to conversational structure.


Huh? exhibits linguistic conventions that speakers need to learn in order to use the form properly. A learner of Spanish has to know that repair is initiated with the mid front unrounded vowel “e↗”, a learner of Cha'palaa has to know that the form is more like “a” with falling intonation, and a learner of Dutch has to know that a glottal fricative at onset is common: “h”. Its acquisition follows a normal trajectory, at least in American English-speaking children [37]. Second language learners' reports confirm that the precise form of this interjection has to be learnt, and that intuitions are not necessarily a reliable guide in this process [38].

Perhaps there is a continuum from non-linguistic vocalisations like sneezing and crying to prototypical conventional lexical items like bless you and pain [39]. Our evidence suggests that huh? is more on the word side of that continuum. Based on the fact that huh? is integrated in multiple linguistic subsystems and conventionalised in language-specific ways we conclude that huh? a lexical word.

Is huh? universal?

Although there is systematic calibration to specific language systems, the bandwidth of the variation of OIR interjections across languages is exceedingly narrow. In all languages investigated, it is a monosyllable with at most a glottal onset consonant, an unrounded low front central vowel, and questioning intonation.

Narrow bandwidth of variation.

We have already shown that the uniformity of the interjections is in striking contrast to the question words that languages can recruit for the same function (Table 1). Another way to appreciate the small range of cross-linguistic variation exhibited by this form is to consider it in context of the possibility space for words in spoken languages. Across languages, words can consist of one or more syllables, but the OIR interjection was never longer than one syllable in the languages we have studied, even in those like Murrinh-Patha, for which phonological words are generally longer than one syllable. Across languages, syllables can have rich internal structure, but the only structure attested in the OIR interjection is (C) V, i.e. a vowel V with an optional onset consonant C, even in languages like Dutch, where CVC syllables are common.

Similarity in vowels and consonants.

Strong constraints on variation are also seen in the vowels and consonants employed. Vowel space can be depicted as a two-dimensional plane formed by height and backness. On average, languages have around 6 vowel phonemes [40], which tend to be maximally spread across this space to increase perceptual distinctiveness [41]. Given this fact, it is striking that the vowels of OIR interjection tokens are only found in the low front central corner of vowel space (Figure 2), and that on a third dimension of lip rounding OIR interjections are only found on the ‘unrounded’ side. Consonants are articulated at different locations throughout the vocal tract (lips, teeth, alveolar ridge, palate, velum, uvula, pharynx, epiglottis) and with different manners of articulation, from plosives, nasals and trills to taps, fricatives, and glides – a multidimensional possibility space in which the International Phonetic Alphabet records at least 64 simple phonemic consonants (and three times as many complex variants) attested in the world's languages [4]. Out of this enormous range of possibilities, only two basic sounds, the glottal consonants [ʔ] and [h], are found in the OIR interjection across languages.

Such limited variation and striking similarity across languages is wholly unexpected on the basis of the principle of the arbitrariness of the sign. Does this mean that huh? is a universal word? We propose a qualified yes. Qualified, because huh? is clearly not phonetically the same word across languages – if Cha'palaa tokens were cross-spliced into Spanish dialog, Spanish speakers would likely be confused. What appears to be universal is the function of this interjection along with a set of constraints determining its form. Other-initiated repair sequences have been found in all languages investigated so far, and no language appears to lack an interjection for this function. Thus huh? is universal in the sense that a short, questioning interjection like it with the function of initiating repair is likely to be attested in all natural spoken languages.

Possible explanations

As we have seen, huh? is so common as to be practically universal, and yet calibrated to specific language systems such that it qualifies as a word. The language-specific nature of words is of course expected; it is the strong similarity that is in need of an explanation. Why do we find basically the same form –something like huh?– everywhere and not, say, bi in one language and rororo in the next? We consider two possible explanations. The first is that huh? is similar across languages because it is an innate grunt. The second is that it is similar as a result of convergent evolution. Empirical evidence supports the second.


On one account, huh? may be similar across languages because it is a natural symptom with a biological basis, like laughs and screams – a “grunt” [8], [10]. Such qualifications, common in the wider literature on interjections, place huh? in a position close to instinctive cries [7], [42]. This would be one explanation for its similarity: it is innate, therefore all humans share it, therefore it assumes roughly the same form in all languages. This view is as hard to support as it is to discount, but we note four doubts.

No known phylogenetic precursor.

Whereas laughter and pain cries (and by extension the conventionalized interjections associated with them) have demonstrable phylogenetic precursors in other mammals [43][46], there is, to the best of our knowledge, no evidence for an animal precursor of huh?. Nor is it obvious what the function and biological survival value of this precursor would be in primates which lack the kind of shared intentionality that underlies human cooperative communication [47], [48].

Not an involuntary response.

Grunts and other non-linguistic vocalisations such as sneezes and pain cries are typically direct, involuntary responses to stimuli [49]. In contrast, the OIR interjection is selected for a specific purpose at a specific juncture in conversation from a larger system of alternative formats for initiating repair [15], [50]. A greater degree of agency over utterance and selection is characteristic of linguistic rather than instinctive expressions.

Acquired like a normal word.

Whereas non-linguistic vocalisations like sneezes, cries and smiles are present at birth or develop soon after [51], [52], the acquisition of huh? follows a trajectory that is very similar to that of other linguistic items. In American English-speaking children, it is employed and responded to somewhat erratically at 2.5 years but perfectly at 5 years [37]. Related to this, the variability of laughs and screams appears to be much greater than what we find for huh?, and is not as strongly regimented by language [44], [53][55].


In terms of evolutionary history, language is a recent arrival that shows clear signs of being a bio-cultural hybrid: a complex adaptive system in continuous cultural evolution within a landscape of cognitive, cultural, and communicative factors [56][59]. Although some of our linguistic abilities are no doubt underpinned by genetic infrastructure, positing innateness for specific linguistic items would hardly be realistic given the timescale involved. Strong cultural universals do not necessarily imply strong innate biases [60] and strong innate biases are unlikely to evolve in cultural systems [61]. If there is a mechanism that can explain cross-linguistic similarity on a more proximate timescale, without resorting to genetic encoding, this is preferred on scientific principles of parsimony.


A more plausible mechanism for the cross-linguistic similarity of huh? is convergent cultural evolution. This proposal sees huh? not as an arbitrary grunt but as a product of cultural evolution in the adaptive context of its interactional environment. The basic principle is well-known from biology: similar environmental constraints have led to the independent evolution of similar body plans in sharks and dolphins, and in the placental mammals of North-America and the marsupials of Australia. Likewise, we propose that the similarity of huh? in unrelated and distantly related languages is due to the fact that it is found in a strongly similar environment in all these languages. What is this environment like?

Conversations are built out of sequences of communicative moves between speakers [62], [63]. These moves –or ‘turns at talk’– are allocated in systematic ways and bear special sequential relations to each other [64], [65]. For instance, a question sets up an expectation that the addressee will provide a fitted response –in this case an answer– in the next move. Speakers inspect moves for their fittedness and aim to minimize gaps and overlaps between them. Speaker change most often takes only between 100–300 milliseconds, and deviations from the timing target can be treated as problematic [66], [67]. In order for this tight timing to work, planning a next turn often has to start well before the end of the preceding turn [68], [69]. Trouble in hearing or understanding is a regular feature of conversation [5], [47]. In the case of such trouble, planning and producing a fitted and timely response will be harder (indeed at times impossible), but the pressure to produce one will be just as strong. Given these pressures of turn-taking and formulation in conversation, a signal that indicates trouble should be minimal and easy to deploy. At the same time, given the communicative importance of indicating trouble (which if not solved might derail the conversation), such a signal should also clearly indicate a knowledge deficit and push for a response. These requirements are met rather precisely in the combination of minimal effort and questioning prosody that characterises the OIR interjection across languages.

Minimal effort.

Many of the formal aspects of the OIR interjection minimize articulatory effort. The codaless monosyllable is the least marked syllable type across languages [68], [70]. The glottal onset, where present, is simply some constriction at the narrowest place in the vocal tract, and the unrounded low front central vowel is close to the neutral state of the articulators – both requiring minimal encoding, planning, and articulation [34]. Additionally, for Spanish phonetic corpus studies show that the vowel target of the interjection is the most frequently attested vowel [71], making retrieval, planning, and production easier [72]. These features render the OIR interjection well-fitted to the interactional environment of other-initiated repair. For the person initiating repair, the OIR interjection is quickly deployable from intention to articulation [68], and therefore easy to produce even under conditions of cognitive duress. For the addressee, the minimal form is a word that is unlike most content words and therefore –by Darwin's principle of antithesis [45]– a good signal that the other has no contentful response on offer.

Questioning prosody.

If ‘minimal’ were the only design requirement, the most low-effort form possible would be enough. But to carry out the work of initiating repair, the OIR interjection also has to signal a knowledge deficit and indicate that a response is needed. We have seen that the intonation of the interjection appears to be calibrated to local systems of questioning prosody. In many languages this means that it has rising intonation – a contour that requires more effort than falling intonation [73], and (in English-speaking infants) has been shown to elicit greater attention [74]. In Cha'palaa and Icelandic, where the OIR interjection has falling intonation, it has a low central unrounded vowel – the vowel that is inherently most sonorous and acoustically salient due to the wide open oral cavity [75]. We propose that the questioning prosody and the acoustic salience of the interjection render it more adaptive for the function of OIR. As a question word devoid of semantic content, it expediently returns the floor to the original speaker and signals that there is trouble to be fixed.

In effect, huh? is an easy to produce, maximally underspecified question word – a tight fit of form and function found in language after language. We propose that this is the result of convergent cultural evolution: the interactional environment of other-initiated repair, present in every language investigated so far, provides a set of selective pressures that pull the interjection towards a similar form and that keep regular processes of language change from affecting the item. This process of convergent evolution explains the narrow bandwidth of the variation, but also the language-specific calibration of the items. To minimize articulatory effort, the OIR interjections of different languages will end up in the same low-effort area of the phonetic possibility space; yet to be recognised as questioning expressions, they will be calibrated to local phonological and prosodic subsystems.

We use ‘convergent evolution’ as a general term for the independent evolution of similarities in form and function. When ancestral forms are known, a distinction can be made between form/function convergence in species that are closely related (‘parallel evolution’) versus in species that are not closely related (‘convergent evolution’). However, this distinction is not always consistently made in biology and recently there have been proposals to use ‘convergent evolution’ as a general term [76]. We use the term in this general sense. Our proposal accounts for the present-day cross-linguistic similarity of huh?, but has to remain agnostic as to its ultimate origins – in the absence of historical language data it is impossible to tell whether the present-day forms go back to one ancestral form (a stabilising evolution scenario [77]) or whether they arose independently in different languages (an independent convergent evolution scenario [78]). In either case, the selective pressures are the same.

The convergent evolution proposal explains the forms documented so far, but also generates the prediction that in undescribed languages as well as newly emerging ones, we can expect to find a similar repair initiator that minimizes articulatory effort while making use of questioning prosody. Independently emerged sign languages of the deaf, though in a different expressive modality (visual-only instead of audio-visual), provide a good test case. Consistent with our proposal, in Argentinian Sign Language, repair can be initiated with a minimal sign that involves a raising of the eyebrows, the semiotic equivalent of questioning prosody [14],[79].

Conversational infrastructure and convergence of linguistic form.

Apart from its explanatory and predictive value, the convergent evolution proposal offers a more general mechanism. For most words in most languages, there is no necessary connection between form and function. This is why words can change over time, and why we expect even words with similar functions to have different forms in unrelated languages. Accordingly, cultural evolutionary models of language change have tended to depict languages as collections of words evolving in utterances [80], [81], with various social and cognitive biases influencing transmission [82], [83] and with frequency of use as a primary factor influencing rates of change and divergence [84], [85]. However, our study points to a factor that may constrain divergence or diachronic drift: the selective pressures of specific conversational environments, which may cause convergent cultural evolution.

The possibility should not be surprising. After all, words evolve in utterances in conversation, so conversational infrastructure is part of the evolutionary landscape for words. We are referring here to the sequential infrastructure that serves as the common vehicle for language use – an infrastructure that may well predate more complex forms of language and that seems largely independent of sometimes radical differences between individual languages [63], [66], [86], [87]. A clear effect of this conversational ecology on the cultural evolution of linguistic items has not, to our knowledge, been observed before.

Though we have focused on huh? as a case study, the mechanism we propose has wider relevance. In our corpora, we have noted other items that are strongly similar in form and function across unrelated languages: continuers like mm/m-hm [88], hesitation markers like uh/um [89], [90], and change of state tokens like oh/ah [91]. It would be neither plausible nor parsimonious to propose that all of these have precursors in distinct innate grunts. Instead, we observe that these interjections all serve important discourse regulatory functions, and we propose that the reason they are so similar across languages is that common communicative needs and conversational infrastructure conspire to create, for each of them, a set of similar selective pressures constraining their evolution. The ultimate fit to the tight constraints of their conversational environments, these words stay put and help us conduct conversation in optimal ways. The approach followed in this study can be systematically extended to the larger set of discourse regulatory expressions and beyond, to explore further effects of conversational ecologies on language structure.


We have presented evidence and arguments that huh?, or more precisely a short questioning interjection with the function of other-initiation of repair, is a universal word likely to be attested in similar form in all natural spoken languages. The similarity of this interjection across languages is unlikely to be specified in our genetic makeup and we argue that it is the result of convergent cultural evolution: a monosyllable with questioning prosody and all articulators in near-neutral position is the optimal fit to the sequential environment of other-initiated repair.

Our proposal invites closer attention to the infrastructure for social interaction that underlies language in use, and its possible influence on language structure. It also enhances existing models of language evolution and change by providing a mechanism for the convergent cultural evolution of linguistic items: conversational environments may exert selective pressure towards the evolution of common optimised forms, calibrated to local linguistic systems. Hence, we see how the investigation of a seemingly banal everyday word –previously characterised as a grunt or dismissed as a non-lexical sound– can shed light on the emergence and motivation of linguistic signs.

Supporting Information

File S1.

Combined Supporting Information containing a description of the auditory analysis, the coding scheme used, and further information related to nasality and mouth aperture.




This work was carried out in the project “Interactional Foundations of Language” within the Language and Cognition Department at the Max Planck Institute for Psycholinguistics. We thank our colleagues (individually credited in Table 2) for making interjection tokens from their corpora available for analysis. We thank Dan Dediu, Carlos Gussenhoven, Sean Roberts, and Joe Blythe for helpful comments on earlier drafts, Mark Sicoli for providing some Meso-American data points, and the PLoS One reviewers for constructive feedback.

Author Contributions

Conceived and designed the experiments: MD FT NJE. Performed the experiments: MD FT NJE. Analyzed the data: MD FT NJE. Wrote the paper: MD NJE FT. Performed the instrumental analysis: FT.


  1. 1. De Saussure F (1983) Course in General Linguistics. La Salle, IL: Open Court.
  2. 2. Hockett CF (1960) The Origin of Speech. Scientific American 203: 89–96.
  3. 3. Ladefoged P, Maddieson I (1996) The Sounds of the World's Languages. Oxford, OX, UK: Blackwell Publishers.
  4. 4. International Phonetic Association (1999) Handbook of the International Phonetic Association: a guide to the use of the International Phonetic Alphabet. Cambridge: Cambridge University Press.
  5. 5. Schegloff EA, Jefferson G, Sacks H (1977) The Preference for Self-Correction in the Organization of Repair in Conversation. Language 53: 361–382.
  6. 6. Hayashi M, Raymond G, Sidnell J, editors (2013) Conversational Repair and Human Understanding. Cambridge: Cambridge University Press.
  7. 7. Müller M (1861) Lectures on the Science of Language. London: Longmans, Green.
  8. 8. Ward N (2006) Non-lexical conversational sounds in American English. Pragmatics & Cognition 14: 129–182 doi:10.1075/pc.14.1.08war.
  9. 9. Mazeland H (1987) A short remark on the analysis of institutional interaction: the organization of repair in lessons.
  10. 10. Schegloff EA (1997) Practices and actions: Boundary cases of other-initiated repair. Discourse Processes 23: 499–545 doi:10.1080/01638539709545001.
  11. 11. Drew P (1997) “Open” class repair initiators in response to sequential sources of trouble in conversation. Journal of Pragmatics 28: 69–101.
  12. 12. Zimmerman DH (1999) Horizontal and Vertical Comparative Research in Language and Social Interaction. Research on Language & Social Interaction 32: 195–203 doi:10.1080/08351813.1999.9683623.
  13. 13. Dingemanse M, Floyd S (in press) Conversation across cultures. In: Enfield NJ, Kockelman P, Sidnell J, editors. Cambridge Handbook of Linguistic Anthropology. Cambridge: Cambridge University Press.
  14. 14. Enfield NJ, Dingemanse M, Baranova J, Blythe J, Brown P, et al.. (2013) Huh? What? – A first survey in 21 languages. In: Hayashi M, Raymond G, Sidnell J, editors. Conversational Repair and Human Understanding. Cambridge: Cambridge University Press.
  15. 15. Dingemanse M, Blythe J, Dirksmeyer T (in press) Formats for other-initiation of repair across languages: An exercise in pragmatic typology. Studies in Language.
  16. 16. Nisbett RE, Wilson TD (1977) Telling more than we can know: Verbal reports on mental processes. Psychological review 84: 231–259.
  17. 17. American Anthropological Association (2004) American Anthropological Association Statement on Ethnography and Institutional Review Boards. Available: Accessed 2013 Sept 30.
  18. 18. Linguistic Society of America (2009) Linguistic Society of America Ethics Statement. Available:​thics_Statement.pdf. Accessed 2013 Sept 30.
  19. 19. Browman CP, Goldstein L (1992) Articulatory phonology: An overview. Phonetica 49: 155–180.
  20. 20. Torreira F, Ernestus M (2010) The Nijmegen corpus of casual Spanish. Proceedings of the Seventh conference on International Language Resources and Evaluation (LREC'10), Valletta, Malta. European Language Resources Association (ELRA). p. 2981.
  21. 21. Boersma P, Weenink D (2013) Praat: doing phonetics by computer. Available: Accessed 2013 Sept 30.
  22. 22. Torreira F, Ernestus M (2011) Realization of voiceless stops and vowels in conversational French and Spanish. Laboratory Phonology 2: 331–353.
  23. 23. Ohala JJ (1983) Cross-language use of pitch: an ethological view. Phonetica 40: 1–18.
  24. 24. Gussenhoven C (2004) The Phonology of Tone and Intonation. Cambridge: Cambridge University Press.
  25. 25. Dehé N (2009) An Intonational Grammar for Icelandic. Nordic Journal of Linguistics 32: 5–34 doi:10.1017/S0332586509002029.
  26. 26. Floyd S (2010) Discourse forms and social categorization in Cha'palaa [PhD thesis]. Austin: University of Texas.
  27. 27. Booij G (1999) The phonology of Dutch. Oxford: Oxford University Press.
  28. 28. Haugen E (1958) The Phonemics of Modern Icelandic. Language 34: 55–88 doi:10.2307/411276.
  29. 29. Pétursson M (1976) Aspiration et activité glottale. Phonetica 33: 169–198 doi:10.1159/000259721.
  30. 30. Enfield NJ (2007) A Grammar of Lao. Berlin: Mouton de Gruyter.
  31. 31. Cheng RL (1966) Mandarin Phonological Structure. Journal of Linguistics 2: 135–158 doi:10.1017/S0022226700001444.
  32. 32. Timberlake A (2004) A reference grammar of Russian. Cambridge: Cambridge University Press.
  33. 33. Blythe J (2009) Doing Referring in Murriny Patha Conversation [PhD dissertation]. Sydney: University of Sydney.
  34. 34. Boersma PPG (1998) Functional phonology: Formalizing the interactions between articulatory and perceptual drives. Holland Academic Graphics/IFOTT. Available: Accessed 2013 Sept 16.
  35. 35. Matisoff JA (1975) Rhinoglottophilia: the mysterious connection between nasality and glottality. In: Ferguson CA, Hyman LM, Ohala JJ, editors. Nasálfest: Papers from a Symposium on Nasals and Nasalization. Stanford: Stanford University. 265–287.
  36. 36. Blevins J (2004) Evolutionary Phonology. Cambridge: Cambridge University Press.
  37. 37. Garvey C (1977) The contingent query: a dependent act in conversation. In: Lewis M, Rosenblum LA, editors. Interaction, Conversation, and the Development of Language. New York: Wiley. 63–94.
  38. 38. Matisoff JA (1994) Tone, intonation, and sound symbolism in Lahu: loading the syllable canon. In: Hinton L, Nichols J, Ohala JJ, editors. Sound Symbolism. Cambridge: Cambridge University Press. 115–129.
  39. 39. Crystal D (1966) The Linguistic Status of Prosodic and Paralinguistic Features. Proceedings of the University of Newcastle-upon-Tyne Philosophical Society. Vol. 1: 93–108.
  40. 40. Maddieson I (2011) Vowel Quality Inventories. In: Dryer MS, Haspelmath M, editors. The World Atlas of Language Structures Online. Max Planck Digital Library. Available: Accessed 2013 Sept 30.
  41. 41. Schwartz JL, Boë LJ, Vallée N, Abry C (1997) The Dispersion-Focalization Theory of vowel systems. Journal of Phonetics 25: 255–286 doi:10.1006/jpho.1997.0043.
  42. 42. Sapir E (1921) Language. New York: Harcourt, Brace.
  43. 43. Preuschoft S (1992) “Laughter” and “Smile” in Barbary Macaques (Macaca sylvanus). Ethology 91: 220–236 doi:10.1111/j.1439-0310.1992.tb00864.x.
  44. 44. Vettin J, Todt D (2005) Human laughter, social play, and play vocalizations of non-human primates: an evolutionary approach. Behaviour 142: 217–240 doi:10.1163/1568539053627640.
  45. 45. Darwin C (1872) The Expression of the Emotions in Man and Animals. London: J. Murray.
  46. 46. Byington ST (1942) Interjections of Pain. American Speech 17: 277–279 doi:10.2307/487200.
  47. 47. Clark HH (1996) Using Language. Cambridge: Cambridge University Press.
  48. 48. Tomasello M, Carpenter M, Call J, Behne T, Moll H (2005) Understanding and Sharing Intentions: The Origins of Cultural Cognition. Behavioral and Brain Sciences 28: 675–691 doi:10.1017/S0140525X05000129.
  49. 49. Goffman E (1978) Response Cries. Language 54: 787–815.
  50. 50. Sidnell J (2006) Repair. In: Verschueren J, Östman J-O, editors. Handbook of Pragmatics. Amsterdam: John Benjamins.
  51. 51. Arlitt AH (1946) Innate responses and tendencies to response: A. Reflex and random activities. Psychology of infancy and early childhood (3rd ed): 106–126. doi:10.1037/13263-004.
  52. 52. Alan L, Waters E (1976) The ontogenesis of smiling and laughter: A perspective on the organization of development in infancy. Psychological Review 83: 173–189 doi:10.1037/0033-295X.83.3.173.
  53. 53. Glenn PJ (2003) Laughter in interaction. New York: Cambridge University Press.
  54. 54. Diller A (1980) Cross-cultural pain semantics. PAIN 9: 9–26 doi:10.1016/0304-3959(80)90025-1.
  55. 55. Ehlich K (1985) The language of pain. Theor Med Bioeth 6: 177–187 doi:10.1007/BF00489662.
  56. 56. Marcus GF, Fisher SE (2003) FOXP2 in focus: what can genes tell us about speech and language? Trends in Cognitive Sciences 7: 257–262 doi:16/S1364-6613(03)00104-9.
  57. 57. Hurford JR (2007) The origins of meaning. Oxford: Oxford University Press.
  58. 58. Beckner C, Blythe R, Bybee J, Christiansen MH, Croft W, et al. (2009) Language Is a Complex Adaptive System: Position Paper. Language Learning 59: 1–26 doi:10.1111/j.1467-9922.2009.00533.x.
  59. 59. Berwick RC, Beckers GJL, Okanoya K, Bolhuis JJ (2012) A Bird's Eye View of Human Language Evolution. Frontiers in Evolutionary Neuroscience 4. doi:10.3389/fnevo.2012.00005.
  60. 60. Kirby S, Dowman M, Griffiths TL (2007) Innateness and culture in the evolution of language. PNAS 104: 5241–5245.
  61. 61. Thompson B, Smith K, Kirby S (2012) Cultural Evolution Renders Linguistic Nativism Implausible. In: Scott-Phillips TC, Tamariz M, Cartmill EA, Hurford JR, editors. The Evolution of Language: Proceedings of the 9th International Conference (EVOLANG9). World Scientific. 227–233.
  62. 62. Goffman E (1981) Forms of Talk. Philadelphia: University of Pennsylvania Press.
  63. 63. Enfield NJ (2013) Relationship Thinking: Agency, Enchrony, and Human Sociality. Oxford: Oxford University Press.
  64. 64. Sacks H, Schegloff EA, Jefferson G (1974) A Simplest Systematics for the Organization of Turn-Taking for Conversation. Language 50: 696–735.
  65. 65. Schegloff EA (2007) Sequence Organization in Interaction: A Primer in Conversation Analysis. Cambridge: Cambridge University Press.
  66. 66. Stivers T, Enfield NJ, Brown P, Englert C, Hayashi M, et al. (2009) Universals and cultural variation in turn-taking in conversation. Proceedings of the National Academy of Sciences 106: 10587–10592 doi:10.1073/pnas.0903616106.
  67. 67. Pomerantz AM (1984) Agreeing and disagreeing with assessments: some features of preferred/dispreferred turn shapes. In: Atkinson JM, Heritage J, editors. Structures of Social Action: Studies in Conversation Analysis. Studies in emotion and social interaction. Cambridge: Cambridge University Press. 57–107.
  68. 68. Levelt WJM (1989) Speaking. Cambridge, MA: MIT Press.
  69. 69. Levinson SC (2013) Action formation and ascription. In: Sidnell J, Stivers T, editors. Handbook of Conversation Analysis. 103–130.
  70. 70. Zec D (2007) The syllable. In: De Lacy PV, editor. The Cambridge Handbook of Phonology. Cambridge: Cambridge University Press. 161–194.
  71. 71. Sandoval AM, Toledano DT, de la Torre R, Garrote M, Guirao JM (2008) Developing a phonemic and syllabic frequency inventory for spontaneous spoken Castilian Spanish and their comparison to text-based inventories 1097–1100.
  72. 72. Goldrick M, Larson M (2008) Phonotactic probability influences speech production. Cognition 107: 1155–1164 doi:10.1016/j.cognition.2007.11.009.
  73. 73. Lieberman P (1967) Intonation, perception, and language. Cambridge: M.I.T. Press.
  74. 74. Sullivan JW, Horowitz FD (1983) The effects of intonation on infant attention: The role of the rising intonation contour. Journal of child language 10: 521–534.
  75. 75. Jong K de, Beckman ME, Edwards J (1993) The Interplay Between Prosodic Structure and Coarticulation. Language and Speech 36: 197–212 doi:10.1177/002383099303600305.
  76. 76. Arendt J, Reznick D (2008) Convergence and parallelism reconsidered: what have we learned about the genetics of adaptation? Trends in Ecology & Evolution 23: 26–32 doi:10.1016/j.tree.2007.09.011.
  77. 77. Hansen TF (1997) Stabilizing Selection and the Comparative Analysis of Adaptation. Evolution 51: 1341–1351 doi:10.2307/2411186.
  78. 78. Schluter D, Clifford EA, Nemethy M, McKinnon JS (2004) Parallel Evolution and Inheritance of Quantitative Traits. The American Naturalist 163: 809–822 doi:10.1086/383621.
  79. 79. Bolinger DL (1983) Intonation and Gesture. American Speech 58: 156–174 doi:10.2307/455326.
  80. 80. Croft W (2000) Explaining Language Change: An Evolutionary Approach. Harlow: Pearson Education Limited.
  81. 81. Baxter GJ, Blythe RA, Croft W, McKane AJ (2006) Utterance selection model of language change. Phys Rev E 73: 046118 doi:10.1103/PhysRevE.73.046118.
  82. 82. Enfield NJ (2008) Transmission biases in linguistic epidemiology. Journal of Language Contact 2: 299–310.
  83. 83. Kirby S, Smith K, Brighton H (2004) From UG to Universals: Linguistic adaptation through iterated learning. Studies in Language 28: 587–607 doi:10.1075/sl.28.3.09kir.
  84. 84. Pagel M, Atkinson QD, Meade A (2007) Frequency of word-use predicts rates of lexical evolution throughout Indo-European history. Nature 449: 717–720 doi:10.1038/nature06176.
  85. 85. Zipf GK (1935) The psycho-biology of language. Boston: Houghton Mifflin.
  86. 86. Schegloff EA (2006) Interaction: The Infrastructure for Social Institutions, the Natural Ecological Niche for Language, and the Arena in which Culture is Enacted. In: Enfield NJ, Levinson SC, editors. Roots of human sociality: Culture, cognition, and human interaction. Oxford: Berg. 70–96.
  87. 87. Levinson SC (2006) The Human Interaction Engine. In: Enfield NJ, Levinson SC, editors. Roots of human sociality: Culture, cognition, and human interaction. Oxford: Berg. 39–69.
  88. 88. Gardner R (1997) The Conversation Object Mm: A Weak and Variable Acknowledging Token. Res on Language & Social Interaction 30: 131–156 doi:_10.1207/s15327973rlsi3002_2.
  89. 89. Clark HH, Fox Tree JE (2002) Using uh and um in spontaneous speaking. Cognition 84: 73–111.
  90. 90. De Leeuw E (2007) Hesitation Markers in English, German, and Dutch. Journal of Germanic Linguistics 19: 85–114 doi:10.1017/S1470542707000049.
  91. 91. Heritage J (1984) A Change of State Token and Aspects of Its Sequential Placement. In: Atkinson JM, Heritage J, editors. Structures of Social Action: Studies in Conversation Analysis. Studies in emotion and social interaction. Cambridge: Cambridge University Press. 299–345.
  92. 92. Moerman M (1977) The Preference for Self-Correction in a Tai Conversational Corpus. Language 53: 872–882.
  93. 93. Suzuki K (2010) Other-Initiated Repair in Japanese: Accomplishing Mutual Understanding in Conversation [PhD dissertation]. Kobe: Kobe University.
  94. 94. Kim K (1993) Other-initiated repair sequences in Korean conversation as interactional resources. Japanese/Korean Linguistics. Stanford: CSLI Publications, Vol. 3: 3–18.
  95. 95. Selting M (1987) Reparaturen und lokale Verstehensprobleme. Oder: Zur Binnenstruktur von Reparatursequenzen. Linguistische Berichte 108: 128–149.
  96. 96. Svennevig J (2008) Trying the easiest solution first in other-initiation of repair. Journal of Pragmatics 40: 333–348 doi:10.1016/j.pragma.2007.11.007.
  97. 97. Beck RM (2011) Bridging the language gap: approaches to Herero verbal interaction as development practice in Namibia.
  98. 98. Matuka YM (1991) The pragmatics of palavering in Kikoongo [PhD dissertation]. Indiana: Ball State University.
  99. 99. Haviland JB (2007) Little rituals in interaction. In: Senft G, Basso EB, editors. Ritual Communication. Oxford: Berg.
  100. 100. Sidnell J (2005) Talk and Practical Epistemology: The Social Life of Knowledge in a Caribbean Community. Amsterdam/; Philadelphia: Johns Benjamins Pub. Co.
  101. 101. Sicoli MA (2012) Zapotec Conversations. Available: openpath = MPI1554638%23. Accessed 2013 Sept 30.
  102. 102. Wickham H, Hofmann H (2011) Product plots. Visualization and Computer Graphics, IEEE Transactions on 17: 2223–2230.