Many parliaments or other legislatures consist of two chambers: an elected lower house, and an upper house or Senate which may be appointed or elected by a different mechanism from the lower house. This style of two houses is called bicameral. Legislatures with only one house are known as unicameral.

The lower house is almost always the originator of legislation, and the upper house is the body that offers the "second look" and decides whether to veto or approve the bills. In the United Kingdom legislation can be originated in either house, but the lower house can ultimately prevail if the two houses repeatedly disagree. In most countries the lower house, regarded as more particularly the representatives of the people, has sole or predominant control over matters to do with finance and taxation.

A parliament's lower house is usually composed of at least 100 members, in countries with populations of over 3 million. The number of seats rarely exceeds 400, even in very large countries. In the United Kingdom however, the lower house (the House of Commons) has 650 members. The upper house customarily has anywhere from 20, 50, or 100 seats, but almost always significantly less than the lower house. In the United Kingdom however, the upper house (the House of Lords) currently has slightly more members than the lower house, and at one time (before the exclusion of most of the hereditary peers) had considerably more.