The Sunnis recognise Four principles of appointing a Caliph:
a) Ijmaa, that is, the cosensus of ophlion of the men of power and position on a certain point.
The agreement of all the followers of the prophet is not necessary, nor even is it essential to secure the consent of all the persons of power and position in the Ummah.
b) Nomination by the previous Caliph.
c) Shura: Selection by Committee.
d) Military power, i.e., if anybody acquires power by military force, he will become a Caliph.
The author of 'Sharh-e-Mawaquif' has explained that when an Imam dies and a person possessing the requisite qualifications claims that office (without the oath of allegience (Bai'at) having been taken for him and without his having been nominated to succeed) his claim to caliphate will be recognized, provided his power subdues the people; and apparently the same will be the case when the new caliph happens to be ignorant or immoral. And similarly when a caliph has thus established himself by superior force and is afterwards subdued by another person, the overpowered caliph will be deposed and the conqueror will be recognized as Imam or Caliph.