Final answer:
The terms of one-third of U.S. Senators expire every two years after the second, fourth, and sixth years, ensuring that the Senate remains a continuous body, with only one-third of its members facing reelection at a time.
Step-by-step explanation:
The question 'The terms of one-third of the members expire when?' refers to the staggered terms of U.S. Senators. According to the Constitution, immediately after their first assembly, senators are divided into three classes. The terms of senators in the first class expire after the second year, the second class after the fourth year, and the third class after the sixth year. This creates a cycle where only one-third of the Senate is up for reelection every two years, maintaining the Senate as a continuous body. Midterm elections, which occur halfway between presidential elections, are an example when voters choose one-third of senators. If a vacancy occurs during a legislative recess, the state executive can make temporary appointments until the state legislature meets again.