A spurious diphthong (or false diphthong) is an Ancient Greek vowel that is etymologically a long vowel but written exactly like a true diphthong Greek, Ancient (to 1453);: ει, ου (ei, ou).
A spurious diphthong has two origins: from compensatory lengthening of short Greek, Ancient (to 1453);: ε, ο (e, o) after deletion of a consonant or contraction of two vowels:
In general, spurious Greek, Ancient (to 1453);: ει, ου contracts from Greek, Ancient (to 1453);: ε, ο + ε, ο, ει, ου. The specific rules are more complex.
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:
Early in the history of Greek, the diphthong versions of ει and ου were pronounced as pronounced as /[ei̯, ou̯]/, the long vowel versions as pronounced as /[eː, oː]/. By the Classical period, the diphthong and long vowel had merged in pronunciation and were both pronounced as long monophthongs pronounced as /[eː, oː]/.
By the time of Koine Greek, ει and ου had shifted to pronounced as /[iː, uː]/. (The shift of a Greek vowel to pronounced as /i/ is called iotacism.) In Modern Greek, distinctive vowel length has been lost, and all vowels are pronounced short: pronounced as /[i, u]/.
Long e and o existed in two forms in Attic-Ionic: Greek, Ancient (to 1453);: ει, ου and Greek, Ancient (to 1453);: η, ω (ē, ō). In earlier Severer[2] Doric, by contrast, only Greek, Ancient (to 1453);: η, ω counted as a long vowel, and it was the vowel of contraction. In later forms of Doric, it contracted to Greek, Ancient (to 1453);: ει, ου. Throughout the history of Doric, compensatory lengthening resulted in Greek, Ancient (to 1453);: η, ω.
"Severe" refers to the sterner-sounding open pronunciation of Greek, Ancient (to 1453);: η, ω pronounced as /[ɛː, ɔː]/, in contrast to the closer Greek, Ancient (to 1453);: ει, ου pronounced as /[eː, oː]/.