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Yer is one of two letters in Cyrillic alphabet, namely ъ (ѥръ, jerŭ) and ь (ѥрь, jerĭ). In the Glagolitic alphabet the respective counterparts of these letters were and . They were originally ultra-short vowels in Slavic languages (including Old Church Slavonic), collectively known as the yers. Bulgarian language is the only modern language that preserves both yers as vowels, including ъ. In all other Slavic languages, the yers disappeared - often influencing the surrounding phones - or have evolved into different vowels. However, many languages using the Cyrillic alphabet have kept them to serve specific ortographic functions.
The back yer (Ъ, ъ, italics Ъ, ъ) of the Cyrillic script, also spelled jer or er, is known as the hard sign in the modern Russian and Rusyn alphabets and as er golyam (ер голям, "big er") in the Bulgarian alphabet. The letter is called back yer in the pre-reform Russian orthography, in Old Russian, and in Old Church Slavonic. Originally the yer denoted an ultra-short or reduced middle rounded vowel.
Its companion is the front yer, now known as the soft sign in Russian and as er malək (ер малък, "small er") in Bulgarian (Ь, ь, italics Ь, ь), which was originally also a reduced vowel, more frontal than the ъ, and which is today used to mark the palatalization of consonants in all of the Slavic languages written in the Cyrillic script, except for Serbian and Macedonian, where it is not used although its traces can be seen in the letters њ and љ.
In the Old Church Slavonic language, the yer was a vowel letter, indicating the so-called "reduced vowel": ъ = *[ŭ], ь = *[ĭ] in the conventional transcription. These vowels stemmed from the Proto-Balto-Slavic short */u/ and */i/ (compare Latin angulus and Old Church Slavonic ǫgъlъ. In all West Slavic languages the yer either disappeared or was transformed into /e/ in strong positions, and in South Slavic languages strong yer reflexes differ widely across dialects.
In Common Slavic, the yers were normal short vowels /u/ and /i/. Havlik's law caused yers in certain positions to be pronounced very weakly (perhaps as ultrashort vowels), and to lose the ability to take the word accent. These weak yers were later dropped, while the strong yers evolved into various sounds depending on the individual language.
For determining whether a yer is strong or weak, it is necessary to break the continuous flow of speech into individual words, or prosodic units (phrases which have only a single stressed syllable, and typically include a preposition or other clitic words). The rule for determining which yers are weak and which are strong is as follows:
- A terminal yer is weak.
- A yer which is followed in the next syllable by a non-reduced vowel is weak.
- The yer in the syllable before one with a weak yer is strong.
- The yer in the syllable before one with a strong yer is weak.
In Russian, for example, the yers evolve as follows:
- Strong yers are fully voiced: ь → е (or ë); ъ → о
- weak yers drop entirely, except that the palatalization from a following ь generally remains.
Simply put, in a string of Old Russian syllables each of which has a reduced vowel, the reduced vowels are in modern Russian alternately given full voicing and drop, and the last yer in this sequence will drop. There are some exceptions to this rule, usually considered to be the result of analogy with other words or other inflected forms of the same word, with a different original pattern of reduced vowels. Modern Russian inflection is therefore at times complicated by the so-called "transitive" (lit. беглые [ˈbʲeɡlɨjə] "fugitive" or "fleeting") vowels, which appear and disappear in place of a former yer. For example (OR = Old Russian; R = Russian):
- OR сънъ /ˈsŭ.nŭ/ → R сон [son] "sleep" (nom. sg.)
- OR съна /sŭˈna/ → R сна [sna] "sleep" (gen. sg.)
- OR угълъ /ˈu.ɡŭ.lŭ/ → R угол [ˈu.ɡəl] "corner" (nom. sg.)
- OR угъла /u.ɡŭˈla/ → R угла [ʊˈɡla] "corner" (gen. sg.)