The Tuscan dialect allows for the diphthongization of originally stressed open vowels [Ĕ] and [Ŏ]. Italian adopts this pattern with words such as "cuore" (previously core), "piece" (originally pede), and "nuovo" (previously novo). Neapolitan is distinct in that it allows for the ascending tonic diphthongization of both open and closed syllables such as the Italian "tempo" becoming "tíempo" in Neapolitan, "porco" becoming "puórco", and "luogo" becoming "luóco."

In relatively rare cases, Neapolitan can also use descending diphthongs in rather free variation with ascending diphthongs. For example, "vuoto" can be pronounced [vwote] or [vuete], by switching between the [je] and [wo] diphthongs. Their use is present among only some speakers of certain social classes as it is a stigmatized form. (Ledgeway, 2009).