Separation of Powers and Checks and Balances
The Framers of the US Constitution had learned from their colonial experience that the best way to prevent power from becoming centralized in a few hands, was to devise a system under which the same officials would not be able to make laws, carry them out, and judge their meaning.
Our constitutional system is based on the notion that the three different branches of government each do different things and have different responsibilities, although their powers also overlap. The legislative branch (Congress) has the power to make laws. The executive branch has the power to administer and enforce those laws. With the development of the notion of "judicial review" by the Supreme Court under Chief Justice John Marshal, the courts got the power to interpret the Constitution and decide whether or not a particular law or executive action was "constitutional."
The branches of government "check" each other so no one branch has so much power that it can totally dominate the others. The president "checks" the legislature by vetoing a law it passes. The Supreme Court checks the other two branches to declaring actions to be invalid. There are also "checks" within a single branch. In the legislature, for instance, the House of Representatives and Senate in Congress can check each other when they think differently about a particular piece of legislation.
The whole system is supposed to be "balanced" through the distribution of power and way the branches work together.
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