Spurious diphthong

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

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

Long e and o existed in two forms in Attic-Ionic: ει, ου and η, ω (ē, ō). In earlier Severer^[7] Doric, by contrast, only η, ω counted as a long vowel, and it was the vowel of contraction.^[8] In later forms of Doric, it contracted to ει, ου. Throughout the history of Doric, compensatory lengthening resulted in η, ω.^[9]

"Severe" refers to the sterner-sounding open pronunciation of η, ω [ɛː, ɔː], in contrast to the closer ει, ου [eː, oː].