Iota subscript

Diacritic mark in the Greek alphabet

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⟩.