A spurious diphthong (or false diphthong) is an Ancient Greek vowel that is etymologically a long vowel but written exactly like a true diphthong ει, ου (ei, ou).
A spurious diphthong has two origins: from compensatory lengthening of short ε, ο (e, o) after deletion of a consonant or contraction of two vowels:
In general, spurious ει, ου contracts from 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 [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 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.