[1] According to Kiril Mirchev, this meant that ⟨a⟩ after ⟨i⟩ in the Thessaloniki dialect (which served as a basis for Old Church Slavonic) mutated into a wide vowel that resembled or was the same as yat (/æ/).

The open articulation of yat (as /æ/ or ja) and the reflexes of Proto-Slavic *tj/*ktĭ/*gtĭ and *dj as ⟨щ⟩ (ʃt) and ⟨жд⟩ (ʒd) have traditionally been considered the two most distinctive phonetic features of Old Bulgarian.

The process affected primarily yat in stressed syllables followed by hard consonant, with multiple examples present in manuscripts from both West and East, e.g. the Grigorovich Prophetologion of the late 1100s (e.g., тꙗло instead of тѣло, "body"), the Tarnovo Gospel of 1273 (e.g, тꙗхъ instead of тѣхъ, "them"), the Strumitsa Apostle of the mid-1200s (e.g., прꙗмѫдро instead of прѣмѫдро, "all-wise"), etc.

[23][20] An opposite process of narrowing of yat into /ɛ/ started in the west in the 1200s, with a first example of consistent replacement of ⟨ѣ⟩ with ⟨є⟩ in Tsar Constantine Tikh's Virgin Charter of the early 1260s.

[26] Thus, those Bulgarian dialects that retained their palatalized consonants remained Yakavian in stressed syllables, whereas those that lost them moved towards Ekavism; unstressed yat, in turn, became /ɛ/ practically everywhere.

Standard Bulgarian's alternation of yat between /ja/ or /ʲa/ in stressed syllable before a hard syllable/consonant and /ɛ/ in all other cases is only characteristic of the Balkan dialects (cf.

The open articulation as /æ/ before hard consonant/syllable has survived only in isolated dialects, e.g., Banat Bulgarian and in clusters along the yat boundary.

The open articulation as ⟨а⟩ after hardened ⟨ц⟩ (/t͡s/) survives as a remnant of former yakavism in a number of western Bulgarian and eastern Macedonian dialects (cf.

From the late 19th century until 1945, standard Bulgarian orthography did not reflect the /ja/ and /ɛ/ alternation and used the Cyrillic letter ⟨ѣ⟩ for both in yat's etymological place.

Consequently, after Bulgaria's occupation by the Soviet Union in 1944 and the installation of a puppet government headed by the communists, ⟨ѣ⟩ was summarily thrown of the Bulgarian alphabet and the spelling changed to conform to the Eastern pronunciation by an orthographic reform in 1945 despite any objections.

[30] After 1989, the elimination of yat from the alphabet has generally been regarded as a violation of the unity of the Bulgarian language,[31] in particular, in right-leaning circles, and nationalistic parties like VMRO-BND have campaigned, unsuccessfully, for its reintroduction.

[32][33] In Russian, written confusion between the yat and ⟨е⟩ appears in the earliest records; when exactly the distinction finally disappeared in speech is a topic of debate.

Some scholars, for example W. K. Matthews, have placed the merger of the two sounds at the earliest historical phases (the 11th century or earlier), attributing its use until 1918 to Church Slavonic influence.

A full list of words that were written with the letter yat at the beginning of 20th century can be found in the Russian Wikipedia.

A few inflections and common words were distinguished in spelling by ⟨е⟩ / ⟨ѣ⟩ (for example: ѣ́сть / е́сть [ˈjesʲtʲ] "to eat" / "(there) is"; лѣчу́ / лечу́ [lʲɪˈt͡ɕu] "I heal" / "I fly"; синѣ́е / си́нее [sʲɪˈnʲe.jɪ], [ˈsʲi.nʲɪ.jɪ] "bluer" / "blue" (n.); вѣ́дѣніе / веде́ніе [ˈvʲe.dʲɪ.nʲjə], [vʲɪˈdʲe.nʲjə] "knowledge" / "leadership").

The retention of the letter without discussion in the Petrine reform of the Russian alphabet of 1708 indicates that it then still marked a distinct sound in the Moscow koiné of the time.

[34] The polymath Lomonosov in his 1755 grammar noted that the sound of ⟨ѣ⟩ was scarcely distinguishable from that of the letter ⟨е⟩,[35] although he firmly defended their distinction in spelling.

[36] A century later (1878) the philologist Grot stated flatly in his standard Russian orthography (Русское правописаніе, Russkoje pravopisanije) that in the common language there was no difference whatsoever between their pronunciations.

A proposal for spelling reform from the Russian Academy of Science in 1911 included, among other matters, the systematic elimination of the yat, but was declined at the highest level.

Émigré Russians generally adhered to the old spelling until after World War II; long and impassioned essays were written in its defense, as by Ivan Ilyin in 1952 (О русскомъ правописаніи, O russkom pravopisanii).

Even in the Soviet Union, it is said that some printing shops continued to use the eliminated letters until their blocks of type were forcibly removed; the Academy of Sciences published its annals in the old orthography until approximately 1924.

[citation needed] The older spelling practice within Russia was ended through government pressure as well as by the large-scale campaign for literacy in 1920s and 1930s, conducted in accordance with the new norm.

For example, the name of the business newspaper Kommersant appears on its masthead with a word-final hard sign, which is superfluous in modern orthography: "Коммерсантъ".

As Ukrainian philologist Volodymyr Hlushchenko notes that initially in proto-Ukrainian tongues yat used to represent /ʲe/ or /je/ which around 13th century transitioned into /i/.

[43] While in the same "Grammar" Pavlovsky states that among Little Russians "yat" is pronounced as /i/ (Ѣ произносится какъ Россїйское мягкое j. на пр: ні́жный, лі́то, слідъ, тінь, сі́но.).

In writing, the diphthong /ie̯/ is represented by the trigraph ⟨ije⟩ – this particular inconsistency being a remnant of the late 19th century codification efforts, which planned to redesign common standard language for Croats and Serbs.

Direct Ikavian, Ekavian and mixed reflexes of yat in Čakavian dialects are a much older phenomenon, which has some traces in written monuments and is estimated to have been completed in the 13th century.

The practice of using old yat phoneme in Glagolitic and Bosnian Cyrillic writings in which Serbo-Croatian was written in the centuries that followed was a consequence of conservative scribe tradition.

E.g. grijati < *grějati, sijati < *sějati, bijaše < *bějaše; but htio : htjela < *htělъ : *htěla, letio : letjela (< *letělъ : *letěla).

If short syllable with yat in the word stem lengthens due to the phonetic or morphological conditions, reflex of /je/ is preserved, e.g. djȅlo – djȇlā, nèdjelja – nȅdjēljā.