A spurious diphthong (or false diphthong) is an Ancient Greek vowel that is etymologically a long vowel but written exactly like a true diphthong (ei, ou).
A spurious diphthong has two origins: from compensatory lengthening of short (e, o) after deletion of a consonant or contraction of two vowels:
In general, spurious contracts from . 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 õù and ÿàwere 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, õù and ÿàhad 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: .
Long e and o existed in two forms in Attic-Ionic: and (ÃÂ, Ã Â). In earlier Severer Doric, by contrast, only counted as a long vowel, and it was the vowel of contraction. In later forms of Doric, it contracted to . Throughout the history of Doric, compensatory lengthening resulted in .
"Severe" refers to the sterner-sounding open pronunciation of , in contrast to the closer .