Congress is a bicameral legislature, meaning that it has two parts: the Senate and the House of Representatives. The Senate contains 100 members, 2 from each state. The House of Representatives is made up of 435 members, each state earning representatives based on population. It is divided this way because some states have very small populations compared to others. In creating two houses, the states with smaller populations have a space where their vote counts as much as any other state (in the Senate), and the states with larger populations have a space where their vote counts based on how many people they represent (in the House).
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