Some come from e-grade of ablaut + i, or o-grade + u, co-existing beside forms with the other grade: 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 time of Koine Greek, ει and ου had shifted to [iː, uː].
Long e and o existed in two forms in Attic-Ionic: ει, ου and η, ω (ē, ō).
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].