Checks and Balances
Power is divided among the three branches of the federal government and the states, each of which checks--that is, restrains--and balances the others. By dividing power, the Constitution pitted the ambitions of one branch against the others to keep any one part of the government from becoming all-powerful and tyrannical. The branches share certain powers but also exercise some exclusive powers.
By Donald Ritchie, Our Constitution (Oxford University Press)