The Czechs' language separated from other Slavic tongues into what would later be called Old Czech by the thirteenth century, a classification extending through the sixteenth century. Its use of cases differed from the modern language; although Old Czech did not yet have a vocative case or an animacy distinction, declension for its six cases and three genders rapidly became complicated (partially to differentiate homophones) and its declension patterns resembled those of Lithuanian (its Balto-Slavic cousin).

By the thirteenth century, what had the Czech language separated from?