Two vowels that produce one sound. As in “aisle”. The “ai” is two letters but one sound.
Greek:
In Greek, the dipthongs are:
| αι as in aisle | αἴρω |
| ει as in eight | εἰ |
| οι as in oil | οἰκία |
| αυ as in sauerkraut | αὐτός |
| ου as in soup | οὐδέ |
| υι as in suite | υἱός |
| ευ, ηυ as in feud | εὐθύς or ηὔξανεν. |
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When ι unites with long α, η or ω to form one sound, the ι is written under the other letter instead of after it, and is called iota subscript. Thus ᾳ, ῃ, ῳ. This iota subscript has no effect whatever upon the pronunciation; ᾳ being pronounced like long α, ῃ like η, ῳ like ω. source