The article deals with the investigation of the half-part counting system in Old Russian. The
half-part counting system is an Indo-European dialectism. In Old Russian it is constituted the word
*polъ
‘half’ and the ordinal numeral in the genitive. The ordinal numeral in this case signified the
anterior to the mentioned number of units. The article proves that the indefinite and definite forms
of the ordinal numeral were strictly differentiated, not as the indefinite and definite forms of
adjectives, which the ordinal numerals erroneosly identified with. In the half-part quantitative only
indefinite (nominal) forms of the numerals were used, and in other cases only definite forms were
used. The half-part quantitative had being preserved in Old Russian sources of different styles and
genres. The changes in it began in the late Old Russian period and they showed themselves in the
influence of other compositive numerals
