Zaiddiyah
Zaiddiyah (also: Zaidi, Zaydi, Zaydiyah, or in the West Fivers) refers to a sect within Shia Islam. Zaidi separated from the main Shia branch (the Shia Ithna Asharia or "Twelvers") over a disagreement as to who the fifth Imam was. Twelvers believe it was Muhammad al-Baqir, while Zaidis hold that it was his half-brother, Zayd bin Ali.
Zaidis see Zayd as the fifth Imam because of the rebellion he led against the Umayyad dynasty, which he believed was corrupt. Muhammad al-Baqir did not engage in political action, whereas Zayd preached that a true Imam must fight against corrupt rulers.
Zaidi beliefs are moderate compared to other Shia sects. The Zaidis do not believe in the infallibility of the Imams, nor that they receive divine guidance. Zaidis also do not believe that the Imamate must pass from father to son, but believe it can be held by any descendant of Ali. They also reject the Twelver notion of a hidden Imam, and like the Ismailis believe in a living imam, or even imams.
In matters of law or fiqh, the Zaidis are actually closest to the Sunni Shafi'i school.
Zaidis form the dominant religious group in Yemen, and the leader of the Zaidi community took the title of Caliph. As such, the ruler of Yemen was known as the Caliph, and this system continued until the middle of the 20th century (1962).
Not All Zaidis believe that Zaid is the true Imam. Zaidis known as Wasitis believe in Twelver Imams and are part of the Shia Ithna Asharia. Most of them settled in India and Pakistan. The biggest group of Zaidis believing in Twelve Shia Imams is known as Saadat-e-Bara. Saadat means descendents of the Prophet Muhammad and Bara means twelve in Hindi and Urdu languages. Saadat-e-Bara's biggest gathering is in Karachi (Pakistan) and Muzaffarnagar (India).