A two-thirds majority means that 2/3 of a voting body is required to agree to something. A simple majority means that one vote more than 1/2 is required. For example, in the U.S. Senate, a 2/3 majority is 67 out of 100 senators, and a simple majority is 51 out of 100. In the case of a tie vote of 50 yes votes and 50 no votes, the Vice President, who is president of the Senate, casts a vote to break the tie.