Article Two of the United States Constitution
This clause talks about Electors and how they choose a President.  Once they are chosen, the Electors meet in their states to vote for the President and Vice President. Originally, candidates only ran for President; there were no candidates for Vice President.  Each Elector voted for two different candidates for President.  They had to vote for at least one candidate who did not live in the Elector's home state.  The candidate who got more than half of the votes became President.  The candidate who got the next highest number of votes (the second-place winner) became Vice President.