Disputes, troubles, dissensions, and strife arose from time to time about the question of succession to the Immamate. The first dynastic trouble arose after the death of the fourth Imam, Zaynul Abidin.
One faction of the Shias accepted Zayd, a son of the fourth Imam as their Imam. Thus, came into existence the first school of law of the Shias. Yemen is the stronghold of the Zaydis. Zayd is the author of the Mqjmu-ul-Flaq, but the work is available only in spurious form.
After the death of Imam Jafar, the sixth Imam, dissensions and dynastic troubles cropped up again. The majority of the Shias followed Musa Kazim. This sector school came to be known as the Ithna Ashari School: it is also known as the Twelvers.
The Ithna Ashari School prevails in Iran. Almost half of the Muslim population of Iraq belongs to this sect. They are also found in Syria, Lebanon and Pakistan. After the Hanafis, the Ithna Ashari sect has the largest number of followers in India. There are numerous works belonging to this school, the most famous being Shar-ai-ul-Islam.
The minority of the Shias, after the death of Jafar, followed Ismail, the elder son of Imam Jafar: consequently, they are known as the Ismailis, and their school is known as Ismaili school.
They are also known as the Seveners. The leading work of the Ismaili school is the Daaimul-Islam. Originally, this school prevailed in Egypt where it found favour with the Fatimidi kings (909-1171 A.D.). It is also known Fatimidi school.
It has small number of followers, in several countries, such as Central Asia, East Africa, South Arabia, Iran, Syria and Pakistan. In India, Ismailis are divided into two groups : (i) the Khojas, they are also known as the Eastern Ismailis, and are the followers of H.H. the Agha Khan who is their 49th Imam, and (ii) the Boharas, who are also known as the Western Ismailis.