What is "iota subscript"

Iota \I*o"ta\, n. [L., fr. Gr. 'iw^ta. See Jot.]

1. The ninth letter of the Greek alphabet ([iota]) corresponding with the English i.

2. [from iota being the smallest letter in the Greek alphabet.] A very small quantity or degree; a jot; a particle.

   They never depart an iota from the authentic formulas of tyranny and usurpation.
   --Burke.

   Iota subscript (Gr. Gram.), iota written beneath a preceding vowel, as a,, h,, w,, -- done when iota is silent.

n. An iota representing a sound that was, in early Greek, pronounced after a long vowel, and persisted in writing long after it dropped out of speech, being written under the long vowel in lowercase.

The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota placed below the letter. It can occur with the vowel letters eta , omega , and alpha . It represents the former presence of an [i] offglide after the vowel, forming a so‐called “long diphthong”. Such diphthongs (i.e., )—phonologically distinct from the corresponding normal or “short” diphthongs (i.e.,  )—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 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 , is known as iota adscript.