Little yus (Ѧ ѧ) and big yus (Ѫ ѫ), or jus, are letters of the Cyrillic script, representing two Common Slavonic nasal vowels in the early Cyrillic and Glagolitic alphabets. Each can occur in iotified form (Ѩ ѩ, Ѭ ѭ), formed as ligatures with the letter Decimal I (І). Other yus letters are blended yus (Ꙛ ꙛ), closed little yus (Ꙙ ꙙ) and iotified closed little yus (Ꙝ ꙝ).
Phonetically, Little yus represents a nasalized front vowel, possibly, while big yus represents a nasalized back vowel, such as IPA . This is also suggested by the appearance of each as a 'stacked' digraph of 'Am' and 'om' respectively.
The names of the letters do not imply capitalization: both little and big yus exist in majuscule and minuscule variants.
All modern Slavic languages that use the Cyrillic alphabet have lost the nasal vowels, making yus unnecessary.
Big Yus was a part of the Bulgarian alphabet until 1945. However, by that time the back nasal was pronounced the same way as ъ . As a result, there were inconsistencies in its usage since people had to rely on memorized orthographic conventions to put it in its etymologically correct place.
There are some Macedonian dialects around Thessaloniki and Kastoria in Northern Greece that still preserve a nasal pronunciation:, 'Where are you going, dear child?').
In Russia, little yus was adapted to represent the iotated /ja/ я in the middle or end of a word; the modern letter я is an adaptation of its cursive form of the seventeenth century, enshrined by the typographical reform of 1708. (This is also why я in Russian often appears as ę in Polish; cf. Russian пять; Polish pięć.) The disappearance of the nasal vowels in Russian has gone further than elsewhere, with many Russian speakers even omitting /n/ in rapid speech in other places (for example, раньше might be realized as райше, or станет as стает).
In Polish, which is a Slavic language written with Latin alphabet, the letter Ę ę has the phonetic value of little yus, while Ą ą has that of big yus. The iotated forms are written ię, ią, ję, ją in Polish. However, the phonemes written ę and ą are not directly descended from those represented by Little and Big Yus, but developed after the original nasals merged in Polish and then diverged again.
Little and big yus can also be found in the Romanian Cyrillic alphabet, used until about 1860. Little yus was used for /ja/ and big yus for /ɨ/.