Article One of the United States Constitution
Section Three provides that the Vice President is the President of the Senate. When serving in this capacity, the Vice President, who is not a member of the Senate, may cast a vote to break a tie.  Early in the nation's history, Vice Presidents frequently presided over the Senate.  In modern times, the Vice President usually does so only during ceremonial occasions or when a tie in the voting is anticipated.  A tie-breaking vote has been cast 243 times by 35 different Vice Presidents.