Like Zaydīsm, Imamism crystallized in the course of the eighth and ninth centuries, but it only acquired its classical form of Twelver Shī‘ism in the tenth and early eleventh. (It could to that extent have been covered in part III of this book rather than here.) Unlike Zaydism, it was not a doctrine for export to the tribal world. It developed in Kufa, Qumm and Baghdad, and to a lesser extent in Medina, where its imams resided until 848, and it reflected the spiritual needs of townsmen who had come to terms with their own exclusion from politics; indeed, it could almost be defined as de-politicized Shi‘īsm. All in all, it differed from Zaydism in four major ways.

First, it was uniformly Rāfiḍī. All Imamis believed ‘Alī to be the Prophet's legatee (waṣī), claiming that the Prophet had publicly designated him as his successor and branding the first three caliphs as usurpers. They would routinely vilify these caliphs (especially the first two) as well as the many Companions who had followed them, branding all of them as infidels or hypocrites. Secondly, where the Zaydīs held all ‘Alī's offspring by Fāṭima to be eligible for the imamate, the Imamis narrowed down the candidates to a single line of Ḥusaynids within which the office was passed down by bequest (waṣiyya), also known as designation (naṣṣ), from father to son, except that al-Ḥasan had been succeeded by his brother al-Ḥusayn.