A yer is one of two letters in Cyrillic alphabets: ъ and ь. The Glagolitic alphabet used, as respective counterparts, the letters and. They originally represented phonemically the "ultra-short" vowels in Slavic languages, including Old Church Slavonic, and are collectively known as the yers. In all modernSlavic languages, they either evolved into various "full" vowels or disappeared, in some cases causing the palatalization of adjacent consonants. The only Slavic language that still uses "ъ" as a vowel sign is Bulgarian, but in many cases, it corresponds to an earlier "ѫ", originally pronounced /õ/. Many languages that use the Cyrillic alphabet have kept one or more of the yers to serve specific orthographic functions. The back yer of the Cyrillic script, also spelled jer or er, is known as the hard sign in the modern Russian and Rusynalphabets and as er golyam in the Bulgarian alphabet. Pre-reform Russian orthography and texts in Old East Slavic and in Old Church Slavonic called the letter "back yer". Originally, it denoted an ultra-short or reduced middle rounded vowel. Its companion, the front yer, now known as the soft sign in Russian, Belarusian and Ukrainian, and as er malək in Bulgarian, originally also represented a reduced vowel, more frontal than the ъ. Today, it marks the palatalization of consonants in all of the Slavic languages written in the Cyrillic script except Serbian and Macedonian, which do not use it at all, but it still leaves traces in the forms of the palatalized letters њ and љ.
In Common Slavic, the yers were normal short vowels /u/ and /i/. Havlik's law caused them, in certain positions, to be pronounced very weakly, perhaps as ultrashort vowels, and to lose the ability to take word accent. The weak yers were later dropped, and the strong yers evolved into various sounds that varied across different languages. To determine whether a yer is strong or weak, one must break the continuous flow of speech into individual words, or prosodic units. The rule for determining weak and strong yers is as follows:
A terminal yer is weak.
A yer followed by a non-reduced vowel in the next syllable is weak.
A yer in the syllable before one with a weak yer is strong.
A yer in the syllable before one with a strong yer is weak.
In Russian, for example, the yers evolved as follows:
Strong yers are fully voiced: ь → е ; ъ → о
Weak yers drop entirely, but 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 their full voicing or drop: the last yer in the sequence drops. There are some exceptions to the 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, complicated by so-called "transitive" vowels, which appear and disappear in place of a former yer. For example :
