[3] In languages such as Amharic, Yoruba, and Zuni, semivowels are produced with a narrower constriction in the vocal tract than their corresponding vowels.
In addition to phonological justifications for the distinction (such as the diphthong alternating with /e/ in singular-plural pairs), there are phonetic differences between the pair:[9] Although a phonological parallel exists between /o̯a/ and /wa/, the production and perception of phonetic contrasts between the two is much weaker, likely because of lower lexical load for /wa/, which is limited largely to loanwords from French, and speakers' difficulty in maintaining contrasts between two back rounded semivowels in comparison to front ones.
[11] Though the spirant approximant is more constricted (having a lower F2 amplitude), longer, and unspecified for rounding (viuda [ˈbjuða] 'widow' vs. ayuda [aˈʝʷuða] 'help'),[12] the distributional overlap is limited.
The two overlap in distribution after /l/ and /n/: enyesar [ẽɲɟʝeˈsaɾ] ('to plaster') aniego [ãˈnjeɣo] ('flood')[13] and although there is dialectal and idiolectal variation, speakers may also exhibit other near-minimal pairs like abyecto ('abject') vs. abierto ('opened').
Other dialects differ in either merging the two or enhancing the contrast by moving the former to another place of articulation ([ʒ]), like in Rioplatense Spanish.