U+0345 COMBINING GREEK YPOGEGRAMMENI

U+0345 was added to Unicode in version 1.1 (1993). It belongs to the block Combining Diacritical Marks in the Basic Multilingual Plane.

This character is a Nonspacing Mark and inherits its script property from the preceding character. It is also used in the script Greek. It is related to its uppercase variant and its titlecase variant . The character is also known as greek non-spacing iota below (1.0) and iota subscript.

The glyph is not a composition. It has a Ambiguous East Asian Width. In bidirectional context it acts as Nonspacing Mark and is not mirrored. The glyph can, under circumstances, be confused with 13 other glyphs. In text U+0345 behaves as Combining Mark regarding line breaks. It has type Extend for sentence and Extend for word breaks. The Grapheme Cluster Break is Extend.

The Wikipedia has the following information about this codepoint:

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.