Original: They are also part of the chain of holders of the office of Caliph, as recognized by most Muslims, the only period in which the Shia Imamate and the Caliphate were united to any degree, excepting the Caliphate of Ali himself.
Simplified: They are also part of the chain of holders of the office of Caliph, as recognized by most Muslims, the only period in which the Shia Imamate and the Caliphate were united to any degree.