Croatian
The Old Croatian yat phoneme is assumed to have a phonetic value articulatorily between the vowels /i/ and /e/. In the Štokavian and Čakavian vowel systems, this phoneme did not have a back vowel parallel; the tendency towards articulatory symmetry led to its merging with other phonemes.
On the other hand, most Kajkavian dialects did have a back vowel parallel (a reflex of *ǫ and *l̥), and both the front and back vowels were retained in these dialects' vowel system before merging with a reflex of a vocalized Yer (*ь). Thus the Kajkavian vowel system has a symmetry between front and back closed vocalic phonemes: */ẹ/ (< */ě/, */ь/) and */ọ/ (< */ǫ/, */l̥/).
Čakavian dialects utilized both possibilities of establishing symmetry of vowels by developing Ikavian and Ekavian reflexes. According to yat reflex Čakavian dialects are divided to Ikavian (mostly South Čakavian), Ekavian (North Čakavian) and mixed Ikavian-Ekavian (Middle Čakavian), in which mixed Ikavian-Ekavian reflex is conditioned by following phonemes according to the Jakubinskij's law (e.g. sled : sliditi < PSl. *slědъ : *slěditi; del : diliti < *dělъ : *děliti). Mixed Ikavian-Ekavian Čakavian dialects have been heavily influenced by analogy (influence of nominative form on oblique cases, infinitive on other verbal forms, word stem onto derivations etc.). The only exception among Čakavian dialects is Lastovo island and the village of Janjina, with Jekavian reflex of yat.
The most complex development of yat has occurred in Štokavian, namely Ijekavian Štokavian dialects which are used as a dialectal basis for modern standard Croatian, and that makes the reflexes of yat one of the central issues of Croatian orthoepy and orthography. In most Croatian Štokavian dialects yat has yielded diphthongal sequence of /ie̯/ in long and short syllables. The position of this diphthong is equally unstable as that of closed */ẹ/, which has led to its dephonologization. Short diphthong has thus turned to diphonemic sequence /je/, and long to disyllabic (triphonemic) /ije/, but that process is not yet completely finished in most Štokavian dialects, so the pronunciation of long yat in Neo-Štokavian dialects can be both monosyllabic (diphthongal or triphthongal) and disyllabic (triphonemic). However, that process has been completed in dialects which serve as a dialectal basis for the codification of Croatian, namely the Western Štokavian dialects with diphthongal value of yat, which is the prescribed orthoepic norm by modern Croatian grammars. 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. This culminated in the Novi Sad agreement and "common" orthography and dictionary. Digraphic spelling of a diphthong as i.e. was used by some 19th century Croat writers who promoted so-called "etymological orthography" – in fact morpho-phonemic orthography which was advocated by some Croatian philological schools of the time, and which was even officialized in the brief period of Independent State of Croatia (1941–45).
Dephonologization of diphthongal yat reflex could also be caused by assimilation within diphthong /ie̯/ itself: if the first part of a diphthong assimilates secondary part, so-called secondary Ikavian reflex develops; and if the second part of a diphthong assimilates the first part secondary Ekavian reflex develops. Most Štokavian Ikavian dialects of Croatian are exactly such – secondary Ikavian dialects, and from Ekavian dialects secondary are the Štokavian Ekavian dialects of Slavonian Posavina and Podravina. They have a common origin with Ijekavian Štokavian dialects in a sense of developing yat reflex as diphthongal reflex.
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 13th century. The practice of using old yat phoneme in Glagolitic and Bosnian Cyrillic writings in which Croatian was written in the centuries that followed was a consequence of conservative scribe tradition.
Reflexes of yat in Ijekavian dialects are from the very start dependent on syllable quantity. As it has already been said, standard Croatian writes trigraph ije at the place of old long yat, which is in standard pronunciation manifested monosyllabically (diphthongally), and writes je at the place of short yat. E.g. bijȇl < PSl. *bělъ, mlijéko < *mlěko < by liquid metathesis from *melkò, brijȇg < *brěgъ < by liquid metathesis from *bȇrgъ, but mjȅsto < *mě̀sto, vjȅra < *vě̀ra, mjȅra < *mě̀ra. There are however some limitations; in front of /j/ and /o/ (< word-final /l/) yat has a reflex of short /i/. In scenarios when /l/ is not substituted by /o/, i.e. not word-finally (which is a common Štokavian isogloss), yat reflex is also different. E.g. grijati < *grějati, sijati < *sějati, bijaše < *bějaše; but htio : htjela < *htělъ : *htěla, letio : letjela (< *letělъ : *letěla). The standard language also allows some doublets to coexist, e.g. cȉo and cijȇl < *cě̑lъ, bȉo and bijȇl < *bě́lъ.
Short yat has reflexes of /e/ and /je/ behind /r/ in consonant clusters, e.g. brȅgovi and brjȅgovi, grehòta and grjehòta, strèlica and strjèlica, etc.
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.
In modern standard Croatian syllables that carry yat reflexes are recognized by alternations in various inflected forms of the same word or in different words derived from the same stem. These alternating sequences ije/je, ije/e, ije/i, ije/Ø, je/i, je/ije, e/ije, e/je, i/ije are dependent on syllable quantity. Beside modern reflexes they also encompass apophonic alternations inherited from Proto-Slavic and Indo-European times, which were also conditioned by quantitative alternations of root syllable, e.g. ùmrijēti – ȕmrēm, lȉti – lijévati etc. These alternations also show the difference between the diphthongal syllables with Ijekavian reflex of yat and syllables with primary phonemic sequence of ije, which has nothing to do with yat and which never shows alternation in inflected forms, e.g. zmìje, nijèdan, òrijent etc.
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