Wikipedia definition
1. Iota subscriptIota subscript in Greek polytonic orthography is a way of writing the letter iota as a small vertical stroke beneath a vowel. It was used in the so-called "long diphthongs" in Ancient Greek, that is, diphthongs the first part of which is a long vowel: ᾳ, ῃ and ῳ. In Latin-based transliteration, it is not usually written. Before the reforms of Eucleides and the adoption of the letters Ω and Η (403-402 BC), there was no way of distinguishing between long and short vowels in Attic Greek.
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