iota subscript
Definition - A small iota written beneath a long vowel. It is used to indicate the second element of a diphthong.

Example -
ῃ and ῳ

Etymology -
The English term was taken directly from the Latin iota subscriptum, which was a translation of the Greek hypogegrammeni. The indicator was invented by ancient-Greek librarians at Alexandria who used it as an editorial marking.

Quotation -
"In Greek polytonic orthography [Iota subscript] 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 of which the first part is a long vowel: ᾳ, ῃ and ῳ."
(Source: Wikipedia s.v. Iota subscript)

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