 It is from among them that the imams of Yemen were selected for the next one thousand years.The imams were usually chosen from the offspring of al-Qasim ar-Rassi and more specifically of al-Hadi, but on at least eight occasions they were picked from other lines descending from Muhammad's grandsons Hasan and Husain.Yemen throughout most of that period was only rarely a unified political entity; in fact, what was included within its frontiers varied widely, and it has not been governed consistently or uniformly by any single set of rulers except for brief periods.