Latin
Diphthongs

Classical Latin had several diphthongs. The two most common were ⟨ ae au ⟩. The former pronounced like the 'i' in mine, and the latter like the 'ow' in power. ⟨ oe ⟩ was fairly rare, and ⟨ ui eu ei ⟩ were very rare, at least in native Latin words. There has also been debate over whether ⟨ ui ⟩ is truly a diphthong in Classical Latin, due to its rarity, absence in works of Roman grammarians, and the roots of Classical Latin words (i.e. hui ce to huic, quoi to cui, etc.) not matching or being similar to the pronunciation of classical words if ⟨ ui ⟩ were to be considered a diphthong.

The sequences sometimes did not represent diphthongs. ⟨ ae ⟩ and ⟨ oe ⟩ also represented a sequence of two vowels in different syllables in aēnus "bronze" and coēpit "began", and ⟨ au ui eu ei ou ⟩ represented sequences of two vowels or of a vowel and one of the semivowels /j w/, in cavē "beware!", cuius "whose", monuī "I warned", solvī "I released", dēlēvī "I destroyed", eius "his", and novus "new".

Old Latin had more diphthongs, but most of them changed into long vowels in Classical Latin. The Old Latin diphthong ⟨ ai ⟩ and the sequence ⟨ āī ⟩ became Classical ⟨ ae ⟩. Old Latin ⟨ oi ⟩ and ⟨ ou ⟩ changed to Classical ⟨ ū ⟩, except in a few words whose ⟨ oi ⟩ became Classical ⟨ oe ⟩. These two developments sometimes occurred in different words from the same root: for instance, Classical poena "punishment" and pūnīre "to punish". Early Old Latin ⟨ ei ⟩ usually monophthongized to a later Old Latin ⟨ ē ⟩, to Classical ⟨ ī ⟩.

By the late Roman Empire, ⟨ ae oe ⟩ had merged with ⟨ e ē ⟩. During the Classical period this sound change was present in some rural dialects, but deliberately avoided by well-educated speakers.
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