The United States Constitution
Although the U.S. Constitution importantly sets out the basic rights guaranteed to all citizens, its greatest impact on the American legal system is the division of power among the three branches of government, which creates the boundaries between federal and state law. The legislative, executive and judicial branches of our government each make distinctive contributions to the legal system.
The Constitution directs Congress in the types of laws it can enact. Once those laws are passed, the executive branch (the President) can veto or sign them. The courts then interpret and enforce the laws.