structural:expansion
Metaphony

An early process that operated in all Romance languages to varying degrees was metaphony (vowel mutation), conceptually similar to the umlaut process so characteristic of the Germanic languages. Depending on the language, certain stressed vowels were raised (or sometimes diphthongized) either by a final /i/ or /u/ or by a directly following /j/. Metaphony is most extensive in the Italo-Romance languages, and applies to nearly all languages in Italy; however, it is absent from Tuscan, and hence from standard Italian. In many languages affected by metaphony, a distinction exists between final /u/ (from most cases of Latin -um) and final /o/ (from Latin -ō, -ud and some cases of -um, esp. masculine "mass" nouns), and only the former triggers metaphony.

Some examples: