A yer is either of two letters in Cyrillic alphabets, ъ (ѥръ, jerŭ) and ь (ѥрь, jerĭ). The Glagolitic alphabet used, as respective counterparts, the letters (Ⱏ) and (Ⱐ). They originally represented phonemically the "ultra-short" vowels in Slavic languages, including Old Church Slavonic, and are collectively known as the yers.
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In all modern Slavic 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 (pronounced /ɤ/) is Bulgarian, but in many cases, it corresponds to an earlier ѫ (big yus), originally pronounced /õ/, used in pre 1945 Bulgarian orthography.
Many languages that use the Cyrillic alphabet have kept one or more of the yers to serve specific orthographic 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 ер голям (er golyam, "big er") 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 (Ь, ь, italics Ь, ь), now known as the soft sign in Russian, Belarusian and Ukrainian, and as ер малък (er malak, "small er") 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.
(As a side-note. in the modern Mongolian Cyrillic alphabet, ь has again been used to represent a very short "i"-type vowel.)