Yat or Jat (Ѣ ѣ; italics: Ѣ ѣ) is the thirty-second letter of the old Cyrillic alphabet. Its name in Old Church Slavonic is jěd’ (ѣдь) or iad’ (ꙗдь). In the common scientific Latin transliteration for old Slavic languages, the letter is represented by e with caron: ě (taken from the Czech alphabet).
The yat represented a Common Slavic long vowel. It is generally believed to have represented the sound, which was a reflex of earlier, or . That the sound represented by yat developed late in the history of Common Slavic is indicated by its role in the second palatalization of the Slavic velar consonants. Significantly, from the earliest texts, there was considerable confusion between the yat and the Cyrillic iotified a (ꙗ). One explanation is that the dialect of Thessaloniki (on which the Old Church Slavic literary language was based) and other South Slavic dialects shifted from /æː/ to /ja/ independently from the Northern and Western branches. The confusion was also possibly aggravated by the fact that Cyrillic Little Yus (ѧ) looks very similar to the older Glagolitic alphabet's yat (ⱑ, supported only in Unicode 4.1; image: ). An extremely rare "iotated yat" form (ꙓ) also exists.
In various modern Slavic languages, the yat has reflexed into various vowels. For example, the old Slavic root běl (white) became bel /bʲel/ in Standard Russian (dialectal /bʲal/, /bʲijel/ or even /bʲil/ in some regions), bil /bʲil/ in Ukrainian, bjal in Bulgarian (bel in Western dialects), biel / biały in Polish, bílý in Czech and biely in Slovak. Older, unrelated reflexes of yat exist; for example, old word телѣгы (telěgi, carts) became modern Russian телеги (telegi) but in Serbian it is таљиге (taljige).
As a result of these reflexes, yat no longer represented an independent phoneme, but rather an already existing one, represented by another Cyrillic letter. As a result, children had to memorize by rote where to write yat and where not. Therefore, the letter was dropped in a series of orthographic reforms: in Serbian with the reform of Vuk Karadžić, which was later adopted for Macedonian, in Russian, Belarusian and Ukrainian roughly with the October revolution, and in Bulgarian and Rusyn as late as 1945. The letter is no longer used in the standard modern orthography of any of the Slavic languages written with the Cyrillic script, although it survives in liturgical and church texts written in the Russian recension of Church Slavonic and has, since 1991, found some favor in advertising.