Spurious diphthong

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).¹

Origin

A spurious diphthong has two origins: from compensatory lengthening of short ε, ο (e, o) after deletion of a consonant:²

*mónt-ya or *móntʰ-ya → *mónsa³ (assibilation from palatalization) → Μοῦσα "Muse"
*doter-ya⁴ → δότειρα "giver" (feminine; compare masculine δοτήρ)

or contraction of two vowels:

φιλέ-ετε → φιλεῖτε "you (pl.) love"
νόος → νοῦς "mind"

In general, spurious ει, ου contracts from ε, ο + ε, ο, ει, ου. 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:

λείπω "I leave" (e-grade: genuine diphthong) — λέ-λοιπα "I have left" (o-grade)
*eleútʰ-somai → ἐλεύσομαι "I will come" (e-grade) — Homeric εἰλ-ήλουθα "I have come" (o-grade)⁵⁶
Proto-Greek *akouyō → ἀκούω "I hear"

Pronunciation

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].

Other dialects

Long e and o existed in two forms in Attic-Ionic: ει, ου and η, ω (ē, ō). In earlier Severer⁷ Doric, by contrast, only η, ω counted as a long vowel, and it was the vowel of contraction.⁸ In later forms of Doric, it contracted to ει, ου. 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].

References

Smyth. A Greek Grammar for Colleges. § 25: diphthongs.
Smyth. A Greek Grammar for Colleges. § 6: genuine and spurious ei, ou.
Smyth. A Greek Grammar for Colleges. § 113: τι̯, θι̯ → σ(σ).
Smyth. A Greek Grammar for Colleges. § 221: -ya in short-a feminine.
ἔρχομαι. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
Smyth. A Greek Grammar for Colleges. § 36: vowel-grades.
Herbert Weir, Smyth. "Introduction, Note 3". A Greek grammar for colleges. The sub-dialects of Laconia, Crete, and Southern Italy, and of their several colonies, are often called Severer (or Old) Doric; the others are called Milder (or New) Doric. Severer Doric has η and ω where Milder Doric has ει and ου.
Smyth. A Greek Grammar for Colleges. § 59.5: Severer Doric εε → η; οο, εο → ω.
Smyth. A Greek Grammar for Colleges. § 37.2: Doric compensatory lengthening.

[1] Smyth. A Greek Grammar for Colleges. § 25: diphthongs.

[2] Smyth. A Greek Grammar for Colleges. § 6: genuine and spurious ei, ou.

[3] Smyth. A Greek Grammar for Colleges. § 113: τι̯, θι̯ → σ(σ).

[4] Smyth. A Greek Grammar for Colleges. § 221: -ya in short-a feminine.

[5] ἔρχομαι. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.

[6] Smyth. A Greek Grammar for Colleges. § 36: vowel-grades.

[7] Herbert Weir, Smyth. "Introduction, Note 3". A Greek grammar for colleges. The sub-dialects of Laconia, Crete, and Southern Italy, and of their several colonies, are often called Severer (or Old) Doric; the others are called Milder (or New) Doric. Severer Doric has η and ω where Milder Doric has ει and ου.

[8] Smyth. A Greek Grammar for Colleges. § 59.5: Severer Doric εε → η; οο, εο → ω.

[9] Smyth. A Greek Grammar for Colleges. § 37.2: Doric compensatory lengthening.

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