Italo‐Romance Metaphony and the Tuscan Diphthongs by Maiden, M

“…The historical causes of general so‐called ‘opening’ diphthongization of proto‐Romance low mid vowels in stressed open syllables are an enduring matter of dispute in historical Romance phonology, the two principal positions being that the diphthongs originate in the assimilatory process of metaphony conditioned by following unstressed vowels, or that they are a matter of ‘spontaneous’ diphthongization associated with lengthening of the vowels. …”