The Sevener reference article from the English Wikipedia on 24-Apr-2004
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Sevener

Ismailis are Shi'a Muslims who consider the Aga Khan their spiritual leader. Some of those who claim that Ismail, the eldest son of Imam Jaffar, was the rightful ruler of all Muslims, are known as "Aga Khanis" or "Ismailis". They are also known as the "Seveners", because Imam Jaffar was the seventh and, according to them, the last Imam. According to the Ismailis, starting from Ali, the eldest son has always inherited the right to rule. The present Aga Khan, Prince Karim, is the 49th direct descendant in a male line down from Ali. His great-grandfather, Hasan Ali Shah, was given the title of Aga Khan by the Sultan of Persia.