Yat or Jat (, ) is the 32nd letter of the old Cyrillic alphabet and name of the sound represented by it. Its name in Old Church Slavonic is (''yět''') or ''ıать'' (''yat'''), in Bulgarian language ''ят'' (''yat''), in Russian language and Ukrainian language ''ять'' (''yat'''), in Serbian language ''јат'' (''yat'', Croatian language spelling ''jat''). In the modern Latin alphabet (Czech language and the common scientific transliteration for old Slavic languages) the letter is represented by "e with caron": .
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 Slavonic is indicated by its role in the second palatalization of the Slavonic velars.
It is significant that from the earliest texts, there is considerable confusion between the ''yat'' and the a iotified (Cyrillic ). One explanation is that the dialect of Thessaloniki, on which the Old Church Slavonic literary language was based, and other South Slavonic dialects shifted from to independent from the Northern and Western branches. The confusion was also possibly aggravated by the fact that in the Glagolitic alphabet ''yat'' ( ) looks very similar to Cyrillic Little Yus ().
[[Image:yat letter russian.JPG|left|thumb|Cyrillic letter ''yat'', set in several fonts. Note that in cursive small yat has quite different shape.]]
In various modern Slavic languages the ''yat'' has reflexed into various vowels. For example, the old Slavic root [běl] (white) became [bel] in standard Russian language (dialectal , or even in some regions), [bil] in Ukrainian language, [bjal] in Bulgarian language, ''biel/biały'' in Polish language, and ''bílý'' in Czech language. Older, unrelated reflexes of yat exist; for example, old word '''' (carts) became modern Russian ''телеги'' but in Serbian it is ''таљиге''.
As a result of these reflexes, "yat" no longer represented an independent phoneme, but rather one identical to that represented by another Cyrillic letter. As a result, children had to memorise by rote where to write ''yat'' and where not. Therefore, the letter was dropped in a series of orthography reforms: in Serbian language with the reform of Vuk Karadžić, which was later adopted for Macedonian language, in Russian language, Belarusian language and Ukrainian language roughly with the October revolution, and in Bulgarian language 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 alphabet, although it survives in liturgical and church texts written in the Russian recension of Church Slavonic language, and has since 1991 found some favour in advertising.
==Yat in Russia and Ukraine==
In the Russian language, confusion between the ''yat'' and e in writing occurs from the earliest records, but when exactly the final disappearance of the original sound from all dialects took place is a topic of scientific debate. Some scholars, for example W.K. Matthews, have placed the coalescence of the two sounds at the earliest historical phases (eleventh century or earlier), attributing its use until 1918 to Church Slavonic influence. Within Russia itself, however, a consensus has found its way into university textbooks of historical grammar (e.g., V.V. Ivanov), that, taking all the dialects into account, the sounds remained predominatly distinct until the eighteenth century, at least under stress, and are distinct to this day in some localities. It may be noteworthy in this respect that the ''yat'' in Ukrainian language usually merged in sound with i, and therefore has remained distinct from e.
The story of the letter ''yat'' and its elimination from the Russian alphabet makes for an interesting footnote in Russian cultural history; see Reforms of Russian orthography. A full list of words that were written with the letter ''yat'' at the beginning of twentieth century can be found in the [http://ru.wikipedia.org/wiki/%D0%AF%D1%82%D1%8C Russian wikipedia.]
==Yat in South Slavic languages==
In the Serbo-Croatian, yat has morphed into three distinct forms: ''e'', ''(i)je'' and ''i'', and this has become one of the differentiating criteria between the dialects. See Serbo-Croatian language#Rendering of yat for details.
==Code positions==
Yat is present in Unicode, though it is often absent from commonly available typeface.
If your font does include it, you should see the capital and small yats here: .