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]:
- Intervocalical /i/ is [j.j], as in peior [pej.jor] ‘worse’; intervocalic /u/ is simple.
- 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’.
- /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.
- 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?