Iota subscript (Greek ὑπογεγραμμένη) 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 of which the first part is a long vowel: ᾳ, ῃ and ῳ. Polytonic orthography for Greek uses a variety of diacritics (πολύ = many + τόνος = accent) to represent aspects of Ancient Greek pronunciation. ... For programming language, see Iota and Jot. ... Ancient Greek phonology is the study of the phonology, or pronunciation, of Ancient Greek. ... In phonetics, a diphthong (Greek δίφθογγος, diphthongos, literally with two sounds) is a vowel combination in a single syllable involving a quick but smooth movement from one vowel to another, often interpreted by listeners as a single vowel sound or phoneme. ...

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. Classical Greek diphthongs ΑΙ, ΕΙ and ΟΙ were composed of a vowel and the letter iota whether or not the vowel was short.

From the mid-fifth to the mid-fourth centuries most of the Greek diphthongs lost their final elements: ηι was pronounced in the same way as ει and both later merged with η; the long αι was pronounced in the same way as the long α and ωι in the same way as ω. This led to the omission of the final ι from later Greek writing. The iota subscript was created when the iota was reintroduced in the Byzantine period to correct the loss of ι in copies of earlier manuscripts, but was placed below rather than beside the vowel to reflect the fact that it was not by then pronounced.

In recent years it has become common for Greek to be quoted and translated with an iota placed beside rather than beneath its vowel.