Hard sign

The letter Ъ ъ (italics Ъ, ъ) of the Cyrillic script is known as er golyam (ер голям – "big er") in the Bulgarian alphabet, as the hard sign (Russian: твёрдый знак, romanized: tvjordyj znak, pronounced [ˈtvʲɵrdɨj ˈznak], Rusyn: твердый знак, romanized: tverdyj znak) in the modern Russian and Rusyn alphabets (although in Rusyn, ъ could also be known as ір), as the debelo jer (дебело їер, "fat er") in pre-reform Serbian orthography, and as ayirish belgisi in the Uzbek Cyrillic alphabet. The letter is called back yer or back jer and yor or jor in the pre-reform Russian orthography, in Old Russian, and in Old Church Slavonic.

Originally the yer denoted an ultra-short or reduced mid rounded vowel. It is one of two reduced vowels that are collectively known as the yers in Slavic philology.