Iota subscript

The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota ⟨ι⟩ placed below the letter. It occurs with the vowel letters alpha ⟨α⟩, eta ⟨η⟩ and omega ⟨ω⟩, and represents the presence of an [i̯] offglide after a long vowel, forming the so‐called "long diphthongs" ⟨ᾳ, ῃ, ῳ⟩. Such diphthongs were phonologically distinct from the "short" diphthongs ⟨ᾰι, ει, οι⟩  of ancient Greek in the pre-classical and classical eras.

The offglide was gradually lost in pronunciation, a process that started already during the classical period and continued during the Hellenistic period, with the result that, from approximately the 1st century BC onwards, the former long diphthongs were no longer distinguished in pronunciation from their corresponding long vowels ⟨ᾱ, η, ω⟩, respectively (the long monophthongs).

During the Roman and Byzantine eras, the iota in such diphthongs, by then mute, was sometimes still written as a standard letter, but was often simply omitted. The iota subscript was invented by Byzantine philologists in the 12th century AD as an editorial symbol, marking the places where such spelling variation occurred. The presence or absence of a mute iota can often help to reduce ambiguity, especially when dealing with inflectional affixes.

When the mute iota is written as a standard letter after its long vowel, as in ⟨αι, ηι, ωι⟩, it is known as an iota adscript. In some editions, these iota adscripts may be reduced in size, as in ⟨αι, ηι, ωι⟩, in order to distinguish them from regular lowercase iotas. In either case, iota adscripts do not carry diacritics, such as breathing marks or accents. In some editions, the iota adscript is used for uppercase letters and the iota subscript for lowercase letters, as in ⟨Αι ᾳ, Ηι ῃ, Ωι ῳ⟩.

In uppercase-only environments, it is typical for the mute iota to be given in full-size when prioritising a faithful rendition of classical texts, as in ⟨ΑΙ, ΗΙ, ΩΙ⟩, but lowercase ⟨Αι, Ηι, Ωι⟩ or reduced ⟨ΑΙ, ΗΙ, ΩΙ⟩ may also occur for the sake of linguistic clarity.