What’s the matter with |U| and |I|? On nasal vowel diphthongization and element asymmetry

In Element-Theory and similar approaches to the internal structure of segments, it is often assumed that the aperture element |A| is more sonorous and different in kind from the coloring elements |I| and |U| (Hulst 2015; Pöchtrager 2006; Schane 1984), while the latter are usually considered to be equally sonorous and display symmetrical behavior. As it has been previously noted, though, this formulation misses a recurrent crosslinguistic asymmetry. |I| and |U| do in fact have distinct behaviors (Carvalho & Klein 1996, Nevins 2012, Veloso 2013, Pimenta 2019), and while typologically, rounding can be absent from a language inventory, “no language has been found that lacks both a front vowel and palatal glide” (Hyman 2008: 100, n. 11). As will be shown, this asymmetry is the source of several phenomena in Portuguese phonology, both synchronic and diachronic. Special attention will be given to nasal vowel diphthongization in non-standard European Portuguese, which reveals a preference for the front offglide over the back offglide, [j] appearing even in some contexts where [w] would be expected. This preference, as will be argued, has its origin in sonority asymmetry, |I| being less sonorous than |U|.