Iota subscript

Iota subscript, Greek Υπογεγραμμένη is an indicator of diphthongs when the stem vowel is long.

Ancient Greek diphthongs ai, ei, oi were composed with a vowel and the letter iota. When original a, e and o were long (α, η, ω), the iota diphthongisation was spelled as ᾳ ῃ and ῳ.

Because of this, the -i element of the diphthong became very vague. While the pronounciation of αι, ει and οι in modern Greek was simplified as e, i and i respectivelly, ᾳ ῃ and ῳ, were simplified as if subscript was absent.

Subscript was then obsoleted along the rest of the polytonic orthography.