The U.S. Congress and almost all the state legislatures are bicameral, meaning that there are two houses through which legislative initiatives must pass. At the federal level, these houses are the U.S. House of Representatives and the U.S. Senate. This system was set up for two reasons:
It was a compromise between those who thought that democratic representation should be based on population (the House) and those who thought that the states should be represented equally (the Senate).
The founding fathers thought that having two houses would limit the number of irrational proposals passed throug