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

Chomsky, N. and Halle, M. 1965. Some controversial questions in phonological theory. Journal of Linguistics 1(2): 97-138.
Clayton, M. L. 1976. The redundance of underlying morpheme-structure conditions. Language 52(2): 295-313.
Cser, A. 2013. Segmental identity and the issue of complex segments. Acta Linguistica Hungarica 60(3): 247-264.
Cser, A. 2020. The Phonology of Classical Latin. John Wiley & Sons.
Devine, A. M. and Stephens, L. D. 1977. Two Studies in Latin Phonology. Anma Libri.
Gorman, K. 2013. Generative phonotactics. Doctoral dissertation, University of Pennsylvania.
Gorman, K. 2014. A program for phonotactic theory. In Proceedings of the Forty-Seventh Annual Meeting of the Chicago Linguistic Society: The Main Session, pages 79-93.
Gouskova, M. and Stanton, Juliet. 2021. Learning complex segments. Language 97(1):151-193.
Hale, M. and Reiss, C. 2008. The Phonological Enterprise. Oxford University Press.
Halle, M. 1975. Confessio grammatici. Language 51(3): 525-535.
Oh, Y., Simon, T., Beckner, C., Hay, J., King, J., and Needle, J. 2020. Non-Māori-speaking New Zealanders have a Māori proto-lexicon. Scientific Reports 10: 22318.
Shibatani, M. 1973. The role of surface phonetic constraints in generative phonology. Language 49(1): 87-106.
Stanley, R. 1967. Redundancy rules in phonology. Language 43(2): 393-436.
Watbled, J.-P. 2005. Théories phonologiques et questions de phonologie latine. In C. Touratier (ed.), Essais de phonologie latine, pages 25-57. Publications de l’Université de Provence.

Let us assume, as I have in the past, that the Classical Latin glides [j, w] are allophones of the short high monophthongs /i, u/. Then, any analysis of this allophony must address the following four asymmetries between [j] and [w]:

1. Intervocalical /i/ is [j.j], as in peior [pej.jor] ‘worse’; intervocalic /u/ is simple.
2. Intervocalically, /iu/ is realized as [jw], as in laeua [laj.wa] ‘left, leftwards’ (fem. nom.sg.), but /ui/ is realized as [wi], as in pauiō [pa.wi.oː] ‘I beat’.
3. /u/ preceded by a liquid and followed by a vowel is also realized as [w], as in ceruus [ker.wus] and silua [sil.wa] ‘forest’, but /i/ is never realized as a glide in this position.
4. There are two cases in which [u] alternates with [w] (the deadjectival suffix /-u-/ is realized as /-w-/ when preceded by a liquid, as in caluus [cal.wus] ‘bald’, and the perfect suffix /-u-/ is realized as /-w-/ in “thematic” stems like cupīuī [ku.piː.wiː] ‘I desired’); there are no alternations between [i] and [j].

What rules gives rise to these asymmetries?

Post-war structuralist phonology greatly emphasized phonemics and largely ignored morphophonemics. But in 1959, Morris Halle’s Sound Pattern of Russian argued that the distinction between allophony and alternation has little cognitive importance, and in fact the distinction leads to an unnecessary duplication of effort. As a result of Halle’s forceful arguments, the contrast between phonemic and morphophonemic processes plays little role in modern phonological theory. I would like to go one step further and suggest that patterns of alternation are actually more principled facts than those of allophony. Simply put, a speaker must command the pattern of alternation in their language; but it is not at all clear whether they exploit allophony when constructing their lexical entries. This is highlighted most clearly by the notions of lexicon optimization, Stampean occultation, and richness of the base in Optimality Theory, though as Hale et al. (1998) note, similar points apply to rule-based theories.

In writing the Romans did not draw distinctions between the high monophthongs [i, u, iː, uː] and glides [j, w], respectively. This naturally led structuralist linguists (e.g., Hall 1946) to suggest that the glides are allophones of the high monophthongs. There are some apparent problems with this suggestion, though not all of them are fatal. One point that has largely been ignored in this discussion is that Classical Latin has at least four types of plausible alternations between high monophthongs and the corresponding glides. In this squib I review these alternations.

Deverbal -u- derivatives

There are a large number of adjectival derivatives formed from verbal stems by the addition of -u- and the appropriate agreement suffixes, e.g., masculine nominative singular (masc. nom.sg.) -u-us, feminine nom.sg. -u-a, and neuter nom.sg. -u-um, and so on. These derivatives have a similar semantics to past participles (“having been Xed”) but in some cases have a secondary meaning “able to be Xed”. For example, the masc. nom.sg. form dīuiduus [diːwi.du.us] means ‘divided’ (cf. dīuidō [diːwi.doː] ‘I divide’) but also ‘divisible’. This is a fairly productive process, as the following examples show. (I have taken the liberty of leaving off certain further productive derivatives, such as intensified adjectives in per-.)

(1) assiduus ‘constant, ambiguus ‘hither and thither’, annuus ‘annual, arduus ‘elevated’, cernuus ‘bowed forward’, circumfluus ‘flowing around’ (refluus ‘ebbing’), cōnspicuus ‘visible’, contiguus ‘neighboring’, continuus ‘continuous’, dīuiduus ‘divided; divisible’ (indīuiduus ‘undivided; indivisible’), exiguus ‘strict’, fatuus ‘foolish’, incaeduus ‘uncut’,  ingenuus ‘indigenous’, irriguus ‘irrigated’, mēnstruus ‘monthly’, mortuus ‘dead’ (dēmortuus ‘departed’, intermortuus ‘decayed’, praemortuus ‘prematurely dead’), mūtuus ‘borrowed’ (prōmūtuus ‘paid in advance’), nocuus ‘harmful’ (innocuus ‘harmless’), occiduus ‘westerly’, pāscuus ‘for pasturing’, perpetuus ‘perpetual’, perspicuus ‘transparent’, praecipuus ‘particular’, prōmiscuus ‘indiscriminate’, residuus ‘remaining’,  riguus ‘irrigated’, strēnuus ‘brisk’, succiduus ‘sinking’, superuacuus ‘superfluous’, uacuus ’empty’, uiduus ‘destitute’

In all the above cases …uus is read [u.us]. However, when the stem ends in a liquid [l, r] …uus is read [wus], indicating that the deadjectival affix is realized as [w].

(2)
a. caluus ‘bald’, fuluus ‘reddish-yellow, tawny’, giluus ‘pale yellow’, heluus ‘honey yellow’
b. aruus ‘arable’, curuus ‘bent’ (incuruus ‘bent’), furuus ‘dark, swarthy’, paruus ‘small’, prōteruus ‘violent’, toruus ‘savage’

It is interesting to note that the contexts where -u- is realized as [w] align with a well-known allophonic generalization (Devine & Stephens 1977: 61., 134f.): a u preceded by a (tautomorphemic) coda liquid or front glide, and followed by a vowel, is realized as [w], as in silua [sil.wa] ‘forest’ or ceruus [ker.wus] ‘deer’, but is realized as a vowel when the preceding consonant is either a nasal, an obstruent, or part of a consonant cluster, as in lituus [li.tu.us] ‘trumpet’ or patruus [pa.tru.us] ‘paternal uncle’.

Two residual issues remain. First, when the verbal stem end in qu [kw], the adjectival derivative is spelled …quus. By the normal rules of spelling this would be read as [kwus], which would suggest that a zero allomorph of the adjectival suffix is selected for here.

(3) aequus ‘equal’, antīquus ‘old’, fallāciloquus ‘falsely speaking’ (fātiloquus ‘prophetic’, flexiloquus ‘ambiguous’, grandiloquus ‘grandiloquent’, magniloquus ‘boastful’, uāniloquus ‘lying’, uersūtiloquus ‘slyly speaking’), inīquus ‘unjust’, longinquus ‘distant’, oblīquus ‘slanting, oblique’, pedisequus ‘following on foot’, propinquus ‘near’, reliquus ‘remaining’

This is consistent with the metrical evidence. For instance in the following verse, aequus must be read as bisyllabic.

(4)
hoc opus hic labor est paucī quōs aequus amāuit (Verg., Aen. 6.129)[ok.ko.pu|sik.la.bo|rest.paw|kiː.kwoː|saj.kwu.sa|maːwit]

Secondly, there are a number of deverbal derivatives in -u-us where the verb form also has a stem-final [w]. In this case we also observe [wus].

(5)
a. cauus [ka.wus] ‘hollowed; hollow’ (concauus ‘hollow’); cf. cauō [ka.woː] ‘I excavate’
b. flāuus [flaː.wus] ‘yellow, gold, blonde’ (sufflāuus ‘yellowish’); cf. flāueō [flaː.we.oː] ‘I am yellow’
c. (g)nāuus [naː.wus] ‘active’ (īgnāuus ‘lazy’); cf. nāuō [naː.woː] ‘I do s.t. enthusiastically’
d. nouus [no.wus] ‘new’; cf. nouō [no.woː] ‘I renew’
e. saluus [sal.wus] ‘safe; well’; cf. salueō [sal.we.oː] ‘I am well’
f. uīuus [wːi.wus] ‘living’ (rediuīuus ‘restored to life’); cf. uīuō [wiː.woː] ‘I live’

This may be another context where the adjectival suffix has a zero allomorph, though it is not clear whether we are looking at the same derivational process as above.

The foregoing discussion leads me to posit a deverbal adjective-forming suffix /-u-/ with two phonologically-predictable allomorphs: [w] before liquids, and zero before [kw] and possibly, [w].

The “third stem”

Schoolchildren learning Latin memorize four forms (or principal parts) of each verb: the first person singular (1sg.) present active indicative (e.g., amō ‘I love’), the present infinitive (amāre ‘to love’), the 1sg. perfect active (amāvī ‘I loved’), and the perfect passive participle (amātus masc. nom.sg. ‘loved). The first two principal parts effectively index the so-called “present stem” of the verb, and the third principal part gives the so-called “perfect stem”. The relationship between the present and perfect stem is often unpredictable. Some perfect stems lengthen a monophthong in the final syllable of the present stem (e.g., legō/lēgī ‘I choose/chose’); some perfect stems omit a post-vocalic nasal in the final syllablem with comcomitant lengthening (uincō/uīcī ‘I win/won’); some are mutated by the addition of a -s- perfect suffix (dīcō/dīxī [diː.koː, diːk.siː] ‘I say/said’); others bear a CV-reduplication prefix, and so on. This has lead some to suggest that the latter two stems are essentially “listed” or “stored” for all verbs. This is, for instance, the position of Lieber (1980:141f., 152f.), but has been disputed by Aronoff (1994: chap. 2) and Steriade (2012), among others, who claim there are many productive regularities in both cases.

The majority of verbs have perfects that consist of the bare verb root, the theme vowel, a high back vocoid perfect suffix, and the appropriate person-number agreement suffixes (e.g., 1sg. -ī-). The perfect suffix is preceded by a theme vowel and as the appropriate agreement suffixes are all vowel-initial, it is always intervocalic. Allophonically, this is a context where [u] is never found but [w] is, and this is what we find here: amāuī [a.maː.wiː] ‘I loved’. This type of perfect is in fact found in all conjugations, and found in the overwhelming majority of 1st (-ā- theme vowel) and 4th conjugation (-ī-) verbs (Aronoff 1994:43f.).

(6)
a. cōnsōlāuī [kon.soː.laː.wiː], portāuī [por.taː.wiː] ‘I carried’
b. dēlēuī [deː.leː.wiː] ‘I destroyed’, plēuī [pleː.wiː] ‘I filled up’
c. cupīuī [ku.piː.wiː] ‘I desired’, petīuī [pe.tiː.wiː] ‘I sought’
d. audīuī [aw.diː.wiː] ‘I listened to’, mūnīuī [muː.niː.wiː] ‘I fortified’

However, there is an alternative formulation in which the theme vowel is omitted,  placing the perfect suffix to the right of a consonant, and in this context it is instead realized as [u]. This type of perfect is also found in all conjugations but is most common in the 2nd (-ē-) conjugation.

(7)
a. domuī [do.mu.iː] ‘I tamed’, uetuī [we.tu.iː] ‘I forbid’
b. docuī [do.ku.iː] ‘I taught’, tenuī [te.nu.iː] ‘I held’
c. rapuī [rap.u.iː] ‘I snatched’, texuī [tek.su.iː] ‘I wove’
d. aperuī [a.pe.ru.iː] ‘I opened’, saluī [sa.lu.iː] ‘I leapt’

Together the patterns in (6-7) account for the vast majority of perfects in all conjugations except the 3rd (itself a grab-bag of etymologically dissimilar verbs).

I propose that the default perfect suffix is /-u-/ and that it undergoes glide formation to [w] in (6), in intervocalic position, a generalization consistent with the allophonic facts. In (7), when adjacent to the verb root, glide formation is blocked. However, the examples in (7) cannot take a “free ride” on any allophonic generalization. As can be seen in (7d), the perfect suffix does not form [l.w, r.w] syllable contact clusters, unlike the adjectival suffix in (5). There is a surfeit of possible analyses for the failure of glide formation in this context: it might be an effect specific to the perfect suffix or to the category of verb, or the result of cyclicity or phase-based spellout. We leave the question open for now.

The “fourth stem”

The form of the perfect passive participle, the fourth principal part, similarly problematic. For many verbs, the perfect passive participle is formed by adding to the verb root a -t- suffix and the appropriate agreement suffixes (e.g., in citation form, the masc. nom.sg. -us), once again sometimes accompanied by lengthening of the stem-final vowel and/or leftward voice assimilation (an exception-less rule of Latin) triggered by the -t- as in (8b).

(8)
a. docuī [do.ku.iː] ‘I teach’, doctus [dok.tus] masc. nom.sg ‘taught’
b. tegō [te.goː] ‘I clothe’, tēctus [tek.tus] masc. nom.sg. ‘clothed’

Two verb roots which end in consonant followed by a high back vocoid and form a -t- perfect passive participle: soluō [solwoː] ‘I loosen; I explain’ and uoluō [wolwoː] ‘I roll’. This places the root-final high back vocoid, by hypothesis /u/, between two consonants, a context where glides are forbidden. The result is solūtus [soluːtus] and uolūtus [woluːtus]. However, it should be noted that this particular pattern is limited to these two verbs and their derivatives, and that the long ū is unexpected unless it reflects stem vowel lengthening (cf. tēctus above).

Synizesis and diaeresis

Latin poetry exhibits variation in glide formation. (The following examples are all drawn from Lehmann 2005). Synizesis, the unexpected overapplication of glide formation in response to the meter, can be seen in the following verse.

(9)
tenuis ubī argilla et dūmōsīs calculus aruīs (Verg., G. 2.180)
[ten.wi.su|biːr.gil|let.duː|mōsīs|kal.ku.lu|sar.wiːs]

In this verse, tenuis ‘thin’ occurs initially, which requires that the first syllable be heavy. The only way to accomplish this is to read it as the bisyllabic [ten.wis] rather than the expected trisyllabic [te.nu.is]. Similarly, in another verse (Verg., Aen. 8.599), abiēte, the ablative singular of abiēs ‘silver fir’, must be read as trisyllabic [ab.jeː.te] rather than the expected [ab.i.eː.te].

On the other hand, the poets also make use of diaeresis, or apparent underapplication of glide formation. For example, siluae, the genitive singular of silua ‘forest’, is in one verse (Hor., Carm. 1.23.4) read as trisyllabic [si.lu.aj] rather than as the expected bisyllabic [sil.waj]. The conditions governing synizesis and diaeresis are not yet well understood, but they constitute further evidence for the close grammatical relationship between [i ~ j] and [u ~ w] in Classical Latin.

Conclusion

We have seen four ways in which the Latin high vocoids alternate between vowels and glides. Together, these four patterns provide indirect evidence for the hypothesis that Latin glides are allophones of the corresponding high vowels, though there are some minor dissociations between patterns of allophony and alternations.

[Earlier writing about Latin glides: Latin glides and the case of “belua”]

References

Aronoff, Mark. 1994. Morphology by itself: stems and inflectional classes. Cambridge: MIT Press.
Devine, Andrew M., and Stephens, Laurence D. 1977. Two studies in Latin phonology. Saratoga: Anma Libri.
Hall, Robert A. 1946. Classical Latin noun inflection. Classical Philology 41(2): 84-90.
Hale, Mark and Kissock, Madelyn, and Reiss, Charles. 1998. Output-output correspondence in Optimality Theory. In Proceedings of WCCFL, pages 223-236.
Halle, Morris. 1959. The sound pattern of Russian. The Hague: Mouton.
Lehmann, Christian. 2005. La structure de la syllabe latine. In Touratier, Christian (ed.), Essais de phonologie latine, pages 157-206. Aix-en-Provence: Publications de l’Université de Provence.
Lieber, Rochelle. 1980. On the organization of the lexicon. Doctoral dissertation, MIT.
Steriade, Donca. 2012. The cycle without containment: Latin perfect stems. Ms., MIT.

Latin texts leave the distinction between high monophthongs [i, u, ī, ū] and glides [j, w] unspecified. This has lead some to suggest that the glides are allophones of the monophthongs. For instance, Steriade (1984) implies that the syllabicity of [+high, +vocalic] segments in Latin is largely predictable. Steriade points out two contexts where high vocoids are (almost) always glides: initially before a vowel (# __ V) and intervocalically (V __ V). In these two contexts, the only complications I am aware of arise from competition between generalizations. For instance, in ūua [uː.wa] ‘grape’ and ūuidus [uː.wi.dus] ‘damp’,  intervocalic glide formation appears to bleed word-initial glide formation. (Or it could be the case that ū is ineligible for glide formation by virtue of its length.) And the behavior of two adjacent high vocoids flanked by vowels is somewhat idiosyncratic: compare naevus [naj.wus] ‘birthmark’ and saeuiō [saj.wi.oː] ‘I am furious’, where (by hypothesis) /ViuV/ surfaces as [j.w], to dēuius [deː.wi.us] ‘devious’ and pauiō [pa.wi.oː] ‘I beat’, where (by hypothesis) /VuiV/ surfaces as [.wi] but never as *[w.j]. And so on.

However, Cser (2012) claims that syllabicity of high vocoids is not at all predictable after a consonant and before a vowel, i.e., in the context C __ V. Here we usually observe [w] when the preceding consonant is coda [j, l, r], as in the aforementioned naevus or silua [sil.wa] ‘forest’. Cser contrasts this latter form with belua ‘wild beast’, which is trisyllabic rather than bisyllabic. However, it is not clear this is a good near-minimal pair. The word was clearly not pronounced as [be.lu.a] because the first syllable scans heavy. In the following hexameter verse, the word comprises the fifth foot, a dactyl:

et centumgeminus Briareus, ac belua Lernae (Verg., Aen. 6.287)

Lewis & Short and the Oxford Latin Dictionary both give this word as bēlua [beː.lu.a]. However, it seems much more likely that the word is in fact bellua [bel.lu.a], as it was sometimes written. (Note also that tautomorphemic geminate ll is robustly attested in Latin.) In this case we would expect glide formation to be blocked because the [lw] complex onset is totally unattested, just as Cser predicts from general principles of sonority sequencing. Thus the above verse is:

[et.ken|tũː.ge.mi|nus.bri.a|re.u.sak|bel.lu.a|ler.naj]

As Cser notes, many of the remaining near-minimal pairs occur at morphological boundaries⁠—and thus look to someone with my theoretical commitments as evidence for the phonological cycle—or relate to the complex onsets qu [kw] and su [sw], which might be treated as contour segments underlyingly. But much work will be needed to show that these apparent exceptions follow from the grammar of Latin.

References

Cser, András. 2012. The role of sonority in the phonology of Latin. In Parker, Steve (ed.), The sonority controversy, pages 39-64. Berlin: Mouton de Gruyter.
Steriade, Donca. 1984. Glides and vowels in Romanian. In Proceedings of the Berkeley Lingusitics Society, pages 47-64.