 It existed as a part of a number of different political systems/ruling dynasties between the ninth and sixteenth centuries, after which it became a part of the Ottoman Empire.After Imam al-Hadi's death in 911, his sons took over the imamate in turn, although it was not hereditary but rather elective among the descendants of Muhammad.From the 11th to the early 17th centuries, however, the imams were usually not chosen from the sons of the former imam, but the title rather circulated among the various Rassid branches.