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 can occur with the vowel letters eta 〈η〉, omega 〈ω〉 and alpha 〈α〉. It represents the former presence of an offglide after the vowel, forming a so-called "long diphthong". Such diphthongs, ηι, ωι, αι (ᾱι), phonologically distinct from the corresponding normal ("short") diphthongs ει, οι, αι (ᾰι), were a feature of ancient Greek in the classical era. The offglide was lost in pronunciation during the Hellenistic period, with the result that from approximately the 1st century B.C. onwards the former diphthongs were no longer distinguished from the simple long vowels η, ω, ᾱ respectively. During the Roman and Byzantine eras, the 〈ι〉, now mute, was sometimes still written as a normal letter, but often simply left out. The iota subscript was invented as an editorial symbol marking the places where such spelling variation occurred by Byzantine philologists in the 12th century AD.

The alternative practice, of writing the mute iota as a full-sized letter 〈ι〉, is known as iota adscript.