Early generative phonology had surprisingly little to say about the theory of phonotactics. Chomsky and Halle (1965) claim that English speakers can easily distinguish between real words like brick, well-formed or “possible” nonce words like blick, and ill-formed or “impossible” nonce words like bnick. Such knowledge must be in part language-specific, since, for instance, [bn] onsets are in some languages—Hebrew for instance—totally unobjectionable. But few attempts were made at the time to figure out how to encode this knowledge.

Chomsky and Halle, and later Stanley (1967), propose sequence structure constraints (SSCs), generalizations which encode sequential redundancies in underlying representations.¹ Chomsky and Halle (p. 100) hypothesize that such generalizations might account for the ill-formedness of bnick: perhaps English consonants preceded by a word-initial obstruent must be liquids: thus blick but not bnick. Shibatani (1973) claims that not all language-specific generalizations about (im)possible words can derive from restrictions on underlying representations and must (instead or also) be expressed in terms of restrictions on surface form. For instance, in German, obstruent voicing is contrastive but neutralized word-finally; e.g., [gʀaːt]-[gʀaːtɘ] ‘ridge(s) vs. [gʀaːt]-[gʀaːdɘ] ‘degree(s)’. Yet, Shibatani claims that German speakers supposedly judge word-final  voiced obstruents, as in the hypothetical but unattested [gʀaːd], to be ill-formed. Similar claims were made by Clayton (1976). And that roughly exhausts the debate at the time. Many years later, Hale and Reiss can, for instance, deny that that this kind of knowledge is part of the narrow faculty of language.

Even if we, as linguists, find some generalizations in our description of the lexicon, there is no reason to posit these generalizations as part of the speaker’s knowledge of their language, since they are computationally inert and thus irrelevant to the input-output mappings that the grammar is responsible for. (Hale and Reiss 2008:17f.)

Many years later, Charles Reiss (p.c.) proposed to me a brief thought experiment. Imagine that you were to ask a naïve non-linguist monolingual English speaker to discern whether a short snippet of spoken language was either, say, Māori or Czech. Would you not expect that such a speaker would do far better than chance, even if they themselves do not know a single word in either language? Clearly then, (at least some form of) phonotactic knowledge can be acquired extremely indirectly, effortlessly, without any substantial exposure to the language, and does not imply any deep knowledge of the grammar(s) in question.²

In a broader historical context, though, early generativists’ relative disinterest in phonotactic theory is something of a historical anomaly. Structuralist phonologists, in developing phonemicizations, were at least sometimes concerned with positing phonemes that have a restricted distribution. And for phonologists working in strains of thinking that ultimately spawned Harmonic Grammar and Optimality Theory, phonotactic generalizations are to a considerable degree what phonological grammars are made of.

A phonological theory which rejects phonotactics as part of the narrow language faculty—as do Hale and Reiss—is one which makes different predictions than theories which do include it, if only because such an assumption necessarily excludes certain sources of evidence. Such a grammar cannot make reference to generalizations about distributions of phonemes that are not tied to allophonic principles or to alternations. Nor can it make reference to the distribution of contrast except in the presence of neutralizing phonological processes.

I illustrated this point very briefly in Gorman 2014 with a famous case from Sanskrit (the so-called diaspirate roots); here I’d like to provide more detailed example using a language I know much better, namely Latin. Anticipating the conclusions drawn below, it seems that nearly all the arguments mustered in this well-known case are phonotactic in nature and are irrelevant in a phonotactics-free theory of phonology.

In Classical Latin, the orthographic sequence qu (or more specifically <QV>) denotes the sound [kw].³  Similarly, gu is ambiguously either [gu] as in exiguus [ek.si.gu.us] ‘strict’ or [gw] as in anguis [aŋ.gwis] ‘snake’. For whatever reason, it seems that is gu was pronounced as [gw] if and only if it is preceded by an n. It is not at all clear if this should be regarded as an orthographic generalization, a phonological principle, or a mere accident of history.

How should the labiovelars qu and (post-nasal) gu be phonologized? This topic has been the subject of much speculation. Devine and Stephens (1977) devoted half a lengthy book to the topic, for instance. More recently, Cser’s (2020: 22f.) phonology of Latin reconsiders the evidence, revising an earlier presentation (Cser 2013) of these facts. In fact three possibilities are imaginable: qu, for instance, could be unisegmental /kʷ/, bisegmental /kw/, or even /ku/ (Watbled 2005), though as Cser correctly observes, the latter does not seem to be workable. Cser reluctantly concludes that the question is not yet decidable. Let us consider this question briefly, departing from Cser’s theorizing only in the assumption of a phonotactics-free phonology.

1. Frequency. Following Devine and Stephens, Cser notes that the lexical frequency of qu greatly exceeds that of k and glide [w] (written u) in general. They take this as evidence for unisegmental /kʷ, gʷ/. However, it is not at all clear to me why this ought to matter to the child acquiring Latin. In a phonotactics-free phonology, there is no simply reason for the learner to attend to this statistical discrepancy. 
2. Phonetic issues. Cser reviews testimonia from ancient grammarians suggesting that the “[w] element in <qu> was less consonant-like than other [w]s” (p. 23). However, as he points out, this is trivially handled in the unisegmental analysis and is a trivial example of allophony in the bisegmental analysis. 
3. Geminates. Cser points out that the labiovelars, unlike all consonants but [w], fail to form intervocalic geminates. However, phonotactics-free phonology has no need to explain which underlying geminates are and are not allowed in the lexicon.
4. Positional restrictions. Under a bisegmental interpretation, the labiovelars are “marked” in that obstruent-glide sequences are rare in Latin. On the other hand, under a unisegmental interpretation, the absence of word-final labiovelars is unexpected. However, both of thes observations have no status in phonotactics-free phonology.
5. The question of [sw]. The sequence [sw] is attested initially in a few words (e.g., suāuis ‘sweet’). Is [sw] uni- or bisegmental?  Cser notes that were one to adopt a unisegmental analysis for the labiovelars qu and gu, [sw] is the only complex onset in which [w] may occur. However, an apparently restricted distribution for [w] has no evidentiary status in phonotactics-free phonology; it can only be a historical accident encoded implicitly in the lexicon.
6. Verb root structure. Devine and Stephens claim that verb roots ending in a three-consonant sequence are unattested except for roots ending in a sonorant-labiovelar sequence (e.g., torquere ‘to turn’, tinguere ‘to dip’). While this is unexplained under a bisegmental analysis, this is an argument based on distributional restrictions that have no status in phonotactics-free phonology. 
7. Voicing contrast in clusters. Voicing is contrastive in Latin nasal-labiovelar clusters, thus linquam ‘I will/would leave’ (1sg. fut./subj. act.) linguam ‘tongue’ (acc.sg.). According to Cser, under the biphonemic analysis this would be the only context in which a CCC cluster has contrastive voicing, and “[t]his is certainly a fact that points towards the greater plausibility of the unisegmental interpretation of labiovelars” (p. 27). It is is not clear that the distribution of voicing contrasts ought to be taken into account in a phonotactics-free theory, since there is no evidence for a process neutralizing voicing contrasts in word-internal trisegmental clusters.
8. Alternations. In two verbs, qu alternates with cū [kuː] in the perfect participle (ppl.): loquī ‘to speak’ vs. its ppl. locūtus and sequī ‘to follow’ vs. its ppl. secūtus. Superficially this resembles alternations in which [lv, bv, gv] alternate with [luː, buː, guː] in the perfect participle. This suggests a bisegmental analysis, and since this is based on patterns of alternation, is consistent with a phonotactics-free theory. On the other hand, qu also alternates with plain c [k]. For example, consider the verb coquere ‘to cook’, which has a past participle coctus. Similarly, the verb relinquere ‘to leave’ has a perfect participle relictus, but the loss of the Indo-European “nasal insert” (as it is known) found in the infinitive may suggest an alternative—possibly suppletive—analysis. Cser concludes, and I agree, that this evidence is ambiguous.
9. ad-assimilation. The prefix ad- variably assimilates in place and manner to the following stem-initial consonant. Cser claims that this is rare with qu-initial stems (e.g., unassimilated adquirere ‘to acquire’ is far more frequent than assimilated acquirere in the corpus). This is suggestive of a bisegmental analysis insofar as ad-assimilation is extremely common with [k]-initial stems. This seems to weakly supports the bisegmental analysis.⁵
10. Diachronic considerations. Latin qu is a descendent of the Indo-European *kʷ, one member of a larger labiovelar series. All members of this series appear to be unisegmental in the proto-language. However, as Cser notes, this is simply not relevant for the synchronic status of qu and gu.
11. Poetic licence. Rarely the poets used a device known as diaeresis, the reading of [w] as [u] to make the meter. Cser claims this does not obtain for qu. This is weak evidence for the unisegmental analysis because the labial-glide portion of /kʷ/ would not obviously be in the scope of diaeresis.
12. The distribution of gu. As noted above the voiced labiovelar gu is lexically quite rare, and always preceded by n. In a phonological theory which attends to phonotactic constraints, this is an explanandum crying out for an explanans. Cser argues that it is particularly odd under the unisegmental analysis because there is no other segment so restricted. But in phonotactics-free phonology, there is no need to explain this accident of history.

Cser concludes that this series of arguments are largely inconclusive. He takes (7, 11) to be evidence for the unisegmental analysis, (3, 5, 8, 9) to be evidence for the bisegmental analysis, and all other points to be largely inconclusive. Reassessing the evidence in a phonotactics-free theory, only (9) and (11), both based on rather rare evidence, remain as possible arguments for the status of the labiovelars. I too have to regard the evidence as inconclusive, though I am now on the lookout for diaeresis of qu and gu, and hope to obtain a better understanding of prefix-final consonant assimilation.

Clearly, working phonologists are heavily dependent on phonotactic arguments, and rejecting them as explanations would substantially limit the evidence base used in phonological inquiry.

Endnotes

1. In part this must reflect the obsession with information theory in linguistics at the time. Of this obsession Halle (1975) would later write that this general approach was “of absolutely no use to anyone working on problems in linguistics” (532).
2. As it happens, monolingual English-speaking New Zealanders are roughly as good at discriminating between “possible” and “impossible” Māori nonce words as are Māori speakers (Oh et al. 2020).
3. I write this phonetically as [kw] rather than [kʷ] because it is unclear to me how the latter might differ phonetically from the former. These objections do not apply to the phonological transcription /kʷ/, however.
4. Recently Gouskova and Stanton (2021) have revived this theory and applied it to a number of case studies in other languages. 
5. It is at least possible that that unassimilated spellings are “conservative” spelling conventions and do not reflect speech. If so, one may still wish to explain the substantial discrepency in rates of (orthographic) assimilation to different stem-initial consonants and consonant clusters. 

References

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