Iota subscriptum

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 [i] offglide after the vowel, forming a so‐called "long diphthong". Such diphthongs (i.e., ηι, ωι, ᾱι)—phonologically distinct from the corresponding normal or "short" diphthongs (i.e., ει, οι, 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 BC onwards the former diphthongs were no longer distinguished from the simple long vowels η, ω, ᾱ respectively. During the Roman and Byzantine eras, the iota, now mute, was sometimes still written as a normal letter but was often simply left out. 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 alternative practice, of writing the mute iota as a full-sized letter ⟨ι⟩, is known as iota adscript.

In Greek, the subscript is called (hupogegramménē, the perfect passive participle form of the verb , hupográphō, 'write below'). Analogously, the adscript is sometimes called προσγεγραμμένη, from the verb προσγράφω, 'to write before'.

The Greek names are grammatically feminine participle forms because in medieval Greek the name of the letter iota, to which they implicitly refer, was sometimes construed as a feminine noun (unlike in classical and in modern Greek, where it is neuter.) The Greek terms, transcribed as ypogegrammeni and prosgegrammeni respectively, were also chosen for use in character names in the computer encoding standard Unicode.

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