Twelver Shia jurisprudence has long been efficient, dynamic, and responsive to societal issues and needs. Due to this advantage, it has been able to guide the Shia community through various historical periods, such as times of marginalization or governance, while maintaining adherence to jurisprudential principles. Despite the influence of multiple factors, custom (ʿurf) may be regarded as playing a fundamental role in this regard. This study, relying on library sources and using a descriptive-analytical method, seeks to answer the question of what role custom played in discovering the criteria (milākāt) of rulings from the perspective of Twelver Shia jurists between the 11th and 13th centuries CE. Examining this issue within a historical framework will not only elucidate the views of jurists from this period and clarify the scholarly developments but will also provide the possibility of mapping the fluctuations of related jurisprudential discussions. Additionally, it will assist in identifying opportunities and challenges in this domain. The conditions for the realization of custom, the transformations of customary practices, the discovery of the criteria of rulings through custom, the interrelation between the authority (ḥujjiyyah) of custom and the ruling of reason (ʿaql), and the perspectives of prominent jurists of this period—such as Shaykh al-Mufid, Sayyid al-Murtada, Ibn Idris, Ibn Zuhra, al-Muhaqqiq al-Hilli, and Sayyid Ibn Tawus—are among the key topics discussed in this study. The findings of this research indicate that the authority of custom extends beyond mere validation or the absence of prohibition. Some jurists equated custom with rational consensus (bināʾ al-ʿuqalāʾ) or the practice of rational individuals (sīrah al-ʿuqalāʾ), considering them to have identical effects. Moreover, certain scholars viewed the authority of custom as intrinsic, suggesting that custom, akin to reason, could serve as one of the sources for discovering the criteria of rulings.