This paper will demonstrate that the Twelver Shīʿī positive law was elaborated from legal materials common to its Sunnī counterpart. In previous research, E. Kohlberg and H. Modarressi, who contributed much to the study of formative periods of Shīʿī jurisprudence, just as H. Motzki did with regard to that of the Sunnīs, made it clear that the two legal systems developed along parallel courses in Ḥijāz and Iraq, the two early learning centers of Islamic law. However, they never discussed the positive law itself. Thus, it remains unclear how the Twelvers, the majority party among the Shīʿīs, elaborated their body of law and how it relates to the Sunnī positive law fi nally articulated by the four schools of law. In an attempt to answer this question, we focus on early debates on khiyār al-majlis, i.e. the option either of the contracting parties has to cancel a contract so long as they have not left the place where it was concluded. The view in favor of the option originated in Medina and found support among the aṣḥāb al-ḥadīth (traditionalists), with whom the Shāfi ʿīs and the Ḥanbalīs sided, as against the Ḥanafīs and the Mālikī majority. Twelver Shīʿī jurisprudence was possibly influenced by both sides, but the overwhelming majority followed the Medinese view as attributed to the Imāms (the sixth Imām Jaʿfar al-Ṣādiq in particular) by Kufan Shīʿī or pro-Shīʿī traditionalists, who seemed to have owed much to Sunnī ḥadīth materials, as do the only two prophetic ḥadīths in “the Four Books”—both related by al-Ṣādiq. It appears that a Twelver Shīʿī element regarding the khiyār of the purchaser of animals was upheld by al-Ṣādiq himself, as opposed to his Sunnī Medinese contemporaries who limited the seller’s liability to defects in slaves.