Spurious diphthong

True diphthongs

By contrast, true diphthongs are e or o placed before i or u. Some come from e-grade of ablaut + i, or o-grade + u, co-existing beside forms with the other grade:

• λείπω "I leave" (e-grade: genuine diphthong) — λέ-λοιπα "I have left" (o-grade)
• *eleútʰ-somai → ἐλεύσομαι "I will come" (e-grade) — Homeric εἰλ-ήλουθα "I have come" (o-grade)
• Proto-Greek *akouyō → ἀκούω "I hear"

Pronunciation

Early in the history of Greek, the diphthong versions of ει and ου were pronounced as , the long vowel versions as . By the Classical period, the diphthong and long vowel had merged in pronunciation and were both pronounced as long monophthongs .

By the time of Koine Greek, ει and ου had shifted to . (The shift of a Greek vowel to is called iotacism.) In Modern Greek, distinctive vowel length has been lost, and all vowels are pronounced short: .

Other dialects

Long e and o existed in two forms in Attic-Ionic: ει, ου and η, ω (ē, ō). In earlier Severer Doric, by contrast, only η, ω counted as a long vowel, and it was the vowel of contraction. In later forms of Doric, it contracted to ει, ου. Throughout the history of Doric, compensatory lengthening resulted in η, ω.

"Severe" refers to the sterner-sounding open pronunciation of η, ω , in contrast to the closer ει, ου .