We use historical and contemporary corpus data to trace the grammaticalization of Persian faqare ‘vertebra’ as a numeral classifier from the eleventh century to contemporary Persian. Cognitive Linguistics serves as the theoretical framework for this study, to which we contribute a diachronic exposition of the evolution of a radial category. In this historical process, a lexical item has been gradually grammaticalized, demonstrating that the lexicon and grammar form a continuum. We find that a visual metaphor based on the physical similarity of a line of bones with verses and paragraphs motivates the use of faqare to refer to texts and units thereof. Subsequent semantic extensions result in polysemy and in a shift in the central meaning of faqare from the domain of texts and documents to the domain of legal proceedings and crime, including both objects (such as permits and bonds) and events (such as robberies and murders). This process yields a radial category of meanings, where the most prototypical meanings refer to legal concepts and related semantic fields. Ultimately the range of use of faqare has extended to the point that it can function as a general classifier, albeit with associations to certain contexts and styles.