The history of the formation of the Imāmite madhhab (doctrinal school of law) has been less invested by historians than that of the Sunni legal schools. Notable developments
have, however, taken place in recent decades. This essay examines the various theories advanced by scholars to describe the development of legal hermeneutics and substantive law (fiqh) specific to the Twelver community. Three key periods are analyzed: that of the historical imāms, the masters of Islamic esotericism for some, the first founders of a Shīʿite legal doctrine for others. We then focus on the Bouyid period when a proper Imāmite madhhab was born under the leadership of rationalist Shīʿite scholars, in particular al-Shaykh al-Mufīd (d. 413/1022). The question of the “orthodoxization” of Shīʿism is also posed, by integrating the Sunni principle of ijmāʿ (consensus) but also by adopting, over the centuries, personal opinion (raʾy) and analogy (qiyās). Particular attention is finally paid to the aḥkām al-arbaʿa (Four Precepts) in Imāmite law, which distinguish Twelver legal doctrine from Sunni fiqh.