Spurious diphthong
Introduction 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'').
Origin A spurious diphthong has two origins: from compensatory lengthening of short  (''e, o'') after deletion of a consonant or contraction of two vowels:#* *''mónt-ya'' or *''móntʰ-ya'' → *''mónsa'' (assibilation from palatalization) →  "Muse"#* ''doter-ya'' →  "giver" (feminine; compare masculine )##*  →  "you (pl.) love"#*  →  "mind"In general, spurious  contracts from . The specific rules are more complex.
True diphthongs 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:*  "I leave" (''e''-grade: genuine diphthong) —  "I have left" (''o''-grade)* *''eleútʰ-somai'' →  "I will come" (''e''-grade) — Homeric  "I have come" (''o''-grade)* Proto-Greek *''akouyō'' →  "I hear"
Pronunciation Early in the history of Greek, the diphthong versions of ει and ου were 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, ει and ου had 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: .
Other dialects Long ''e'' and ''o'' existed in two forms in Attic-Ionic:  and  (''ē, ō''). 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  .
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