The Abbassids were ruled by Kings who styled themselves Caliphs
(since all previous rulers of the Moslem Empires had done
similarly. The First Abbassid Caliph is Abu Al-Abbas As-Saffah. He
ruled from 750 CE to 754 CE and was succeeded by the Abbassid
Al-Mansur (there is a different Umayyad Al-Mansur in Spain in the
900s).