The genre of furuq (sg. farq) writing within Islamic law began in the 10th century and became widespread only in the 13th; it blossomed in what is referred to as the post-formative period. A farq fiqhi, a legal distinction, aims at explaining the difference(s) between situations or actions which are apparently identical yet lead to differing legal outcomes. My paper will study the prehistory and early development of the concept of the farq fiqhi and will delineate how the furuq literature helped drive innovation in Islamic law. This was innovation through interpretation, thus it did not manifest itself in either books of legal theory or legal compendia. The study of medieval Islamic law focuses primarily on these two genres, and therefore past scholars have tended to deemphasize the legal change that occurred in the post-formative period. The prehistory of the legal furuq lies in the field of lexicographical furuq literature. This linguistic writing sought to explain the differences between supposed synonyms, therefore negating their synonymity. The debate over the existence of synonyms in Arabic went to the heart of the debate about the nature of the Arabic language. By denying the existence of true synonyms, these scholars also denied the existence of superfluous elements in Arabic and thus affirmed the perfection of the language. I argue that the composition of works on legal distinctions grew directly out of this lexicographical tradition. For this purpose, my paper will study the two earliest extant works in this tradition, those by al-Qadi ‘Abd al-Wahhab al-Baghdadi and ‘Abd Allah ibn Yusuf al-Juwayni. I will analyze these texts in two ways. First, I analyze their organization and structure in order to show how these two works grow directly out of the linguistic furuq tradition. Further, I analyze their understanding and deployment the concept of a farq. I argue that the farq fiqhi does more than merely explain a ‘legal distinction;’ it also creates a space in which jurists can begin to change, adapt, and alter Islamic law. The usage and discussion of such distinctions show the large interpretive work that furuq performed in the development of Islamic law. Jurists employing legal furuq could effect this change through interpretation while maintaining a façade of normative stability and constancy. It is by studying the concept of a farq fiqhi that this aspect of Islamic law emerges and the agency which a ‘distinction’ held is made manifest.