The U.S. Congress is a bicameral (two-house) legislature. Its basic structure grew out of the Great Compromise at the Constitutional Convention. As a result of that compromise, each state is represented in the upper house (or Senate) by two senators, who serve staggered six-year terms; in the lower house (the House of Representatives), states are represented according to their population. Members of the lower house serve two-year terms. In 1929, the total number of representatives was fixed at 435. Whenever the population shifts (as demonstrated by a decennial census), the country’s 435 single-member legislative districts must be reapportioned to reflect the changes and provide equal representation.