Jer (pronounced yer). Originally, the name for two letters of the Glagolitic alphabet and the Cyrillic alphabet: ъ and ь in Cyrillic. The origin of the name is unknown; possibly, it was derived from the jargon name of a bird (a type of swallow) whose shape in flight resembles the shape of the Glagolitic letters. The jers as sounds arose ca 800 AD in Common Slavic from the short u and i respectively. In their subsequent development in Proto-Ukrainian the jers split into weak and strong depending on their position in the word; namely, they became strong if followed in the next syllable by a weak jer. The strong jers developed into o and e respectively, while the weak jers underwent reduction and were lost ca 1150. Thus, the disyllabic words sъnъ (‘sleep’) and dьnь (‘day’) of late Common Slavic became son and den’ in the modern Ukrainian language. The letter ь is still used in the Ukrainian alphabet; not as a vowel, however, but as a ‘soft sign’ to indicate palatalization of the preceding consonant.
Encyclopedia of Ukraine