Federal practice of law originates with the United States Constitution, which grants the Congress the ability to act out legislative acts for certain bounded intentions like regularising interstate commerce. Almost all legislative acts have been written in the United States Code. A lot of legislative acts establish executive branch authorities the ability to produce rules, which are promulgated in the Federal Register and written into the Code of Federal Regularisations. Ordinances commonly also contain the strength of law under the Chevron philosophy. A lot of cases turn on the signifying of a federal legislative act or ordinance, and juridical renditions of such signifying carry legal strength under the precepts of stare decisis.

In the commencement, federal law traditionally centered on spheres where there was an fast accord of power to the federal authorities in the federal Constitution, like the armed forces, finances, foreign affairs (particularly external accords), tariffs, intellectual property (specifically patents of invention and rights of first publication), and postal service. Since the beginning of the twentieth century, ambitious renditions of the Commerce and Spending articles of the United States Constitution have enabled federal law to amplify into regions like air travel, telecoms, railways, pharmaceuticals, antimonopoly, and trademarks. In a few spheres, like air travel and railways, the federal authorities has formulated a comprehensive strategy that preempts literally whole state law, while in others, like family law, a comparatively diminished amount of federal legislative acts (broadly addressing interstate and external positions) interacts with a much greater body of state law. In regions like antimonopoly and trademark, there are potent polices at both the federal and state degrees that coexist with one another. In a smattering of spheres like insurance, Congress has ordained laws explicitly declining to regularise them as long as the states have laws governing them.