The institution was set up by America’s founding fathers as a compromise between allowing Congress and the people to elect the president.
Technically, Americans cast votes on election day for electors, not the candidates themselves.
The electors are mostly elected officials or party functionaries, and are generally unknown to the public.
There are 538 in all, one for each member of Congress, plus three for District of Columbia.
Although Democrat Hillary Clinton secured almost three million more votes from the public, Mr Trump won the majority of electors – 306.
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