Disagreements among Shiites arose mostly concerning the line of succession of the imamate as well as, for many, the nature of the Hidden Imam, i.e., whether or to what extent he was present in this world rather than in a completely removed sphere. Views also differed regarding the possibility or need for political action.
The three most significant branches within Shiism are labeled with numbers—Imami Shiites are Twelvers, Ismailis are Seveners, and Zaydis are Fivers. Even though these numbers refer to the point in the line of Imams where they disagree with other branches of Shiism, the rift did not take place in the immediate aftermath of that respective Imam's death, but developed over a longer period and was then projected back. The inner division of Shiism developed in a manner not unlike majority Sunnism and minority Shiism in Islam: The majority community, Twelver Shiism, became visible only after what turned out to be minority communities had emerged.
The Zaydis were in fact the first distinct group that can be described as Shiite, although it is likely that, as with other religio-political movements, the development into a coherent sect took place over a longer period. The Zaydis are named after Zayd ibn Ali, a half-brother of a grandson of Husayn, who started a rebellion in 739-740. The genealogical criteria for the imamate of the Zaydis are much less strict than those of other Shiites, but they are complemented by a condition peculiar to this branch. Legitimacy of rule depended on political action, which in practice meant rebellion. With this emphasis on improving the here-and-now, there was little space for the Hidden Imam in Zaydism. The imamate is a much more ordinary and continuous form of rule, and the Zaydi Imams are not believed to have any superhuman powers. Zaydism spread mostly in tribal areas and has been inconsistent in its political claims; Zaydi rebels did not always claim the imamate. The only dynastic rule established by Zaydis is that of the Qasimi Imams in Yemen (1598-1851).
A crucial event in the formation of different Shiite sects was the death of the eleventh Imam, Hasan al-Askari, in 874. The period following his death is referred to as al-hayra, perplexity. No suitable successor was identified and claims arose that the Imam had a child by the name of Muhammad who had gone into Occultation (ghayba), a safe sphere which has been variably identified as belonging to this world or a removed metaphysical realm. After an unsuccessful attempt to establish an institution of safirs, i.e., individuals who established a link to the Hidden Imam, the contact to his followers was interrupted in 941 and the Imam withdrew into the Greater Occultation. Those who followed the line of Imams that ended with Hasan's son Muhammad, and who until then had been simply known as Imamis, turned into Twelver Shiites. The line of Imams came to an end, as far as their presence in this world is concerned, and the principles of quietism and messianic hopes crystallized. The vast majority of Shi'is historically and presently belong to this branch of Shia Islam.
The Seveners or Ismailis are the second significant group within Shiism and comprise several branches. Two of these are well-known in the West: knowledge of the medieval so-called "Assassins" and their leader, the "old man of the mountain," existed since the attacks on Crusaders and their appearance in Marco Polo's report. The group owes its grim reputation as reflected in the name, which was a pejorative label, to attacks on high-ranking Sunni leaders during which the attackers sacrificed their lives, allegedly under the influence of drugs. Related to this branch is the line of Aga Khans, the spiritual leaders of the Ismailis in the modern era who also act as international patrons.
The movement derives its name from Ismail, the son of the sixth Imam Jafar al-Sadiq, who died before his father. According to the Twelvers, the succession then passed to Jafar's younger son, Musa al-Kadhim. Seveners/Ismailis, however, recognize Ismail's son Muhammad (d. ca. 800) as the seventh Imam. Although modern scholars disagree on the circumstances, the Ismaili movement emerged as a secret opposition against the ruling Abbasids. Hence, it is not surprising that its origins lie to a great extent in the dark and remain controversial among historians. Little is known about some of the most crucial figures. The historiography of Ismailism faces the additional difficulty that later divisions may often have been projected back in time.