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Shia Muslims or Shiites are a type of follower of Islam, the other main division is the Sunnis, Shiites are the majority in Iran and in a part of Iraq but in the rest of the Islamic world Shiites form a minority. There are some theological differences between the two, the principal one being that Shiites believe only men who are directly descended from Mohammed can rule the faithful. While Sunnis believe Muslim leaders do not need to be descended from Mohammed, both groups expect men to lead. Men believed to be descended from Mohammed are entitled to wear a black turban which to Shiites indicates their leadership qualities.
Proving direct descent
Well until the 1980’s proving paternity was impossible so nobody can be certain that during the very long period of time (over a millennium) since Mohammed no woman has willingly or unwillingly been unfaithful causing a break in the direct line of descent, even if most wives are faithful a few will deviate. Shiites seem to overlook this.
Parallel with Christianity
There are some theological differences between the Roman Catholic Church the Orthodox Churches and the various different Protestants/Protestant groups but a very important point is who does and who does not accept the authority of the Pope.