Article V of the U.S. Constitution talks about the process of modifying the Constitution. According to the article, the process to propose and ratify an amendment can be started by either the US Congress or the state legislatures:
Both the Senate and the House of the U.S. Congress can propose an amendment only if it reaches at least two-thirds of votes in favor. If the proposal is approved, then it goes to the state legislatures OR the state conventions so that it can be voted on. If the amendment is ratified by at least three-fourths of the state legislatures or the state convention, it becomes a law.
When the state legislatures start the process, they first have to propose a National Convention with two-thirds of votes. Once in the National Convention, the states legislature officially propose the amendment and is voted on. If the amendment is ratified by three-fourths of the state legislatures (or conventions in three-fourths of the states), it becomes a law.
The last one and because no one had to much power each branch was kept inline by the other.
The Constitution
It forms a constitutional government
