As the language spread it absorbed local colouring, and scholars distinguish the classical Old Church Slavonic of the earliest manuscripts from the later regional recensions, Bulgarian, Serbian, Russian and others, collectively called Church Slavonic. The Russian recension became the most widespread, fixed by printed liturgical books from the sixteenth century and corrected in the seventeenth under Patriarch Nikon, a revision whose resisters, the Old Believers, still preserve the earlier readings. Church Slavonic remains the principal liturgical language of the Russian, Serbian, Bulgarian and Ukrainian Orthodox churches and of Slavic Greek Catholic communities, chanted in a continuous tradition more than a millennium old. Its weight on the written culture of Orthodox Slavdom is hard to overstate.
Literary Russian is shot through with Slavonicisms, doublets in which the Church Slavonic form carries the elevated sense and the native Russian form the everyday one, so that the liturgical language remains audible inside the modern one. Debates over replacing Slavonic with the vernaculars recur in every Slavic church, and some now permit modern language liturgy, but the older register keeps its hold, prized for its music, its precision and its unbroken association with prayer.