The law of the United States america was originally largely derived from the common law system of English law, 
which was in force at the time of the Revolutionary War. However, the supreme law of the land is the United States Constitution and, under the Constitution's Supremacy Clause, laws enacted by Congress and treaties to which the U.S. is a party. These form the basis for federal laws under the federal constitution in the United States, circumscribing the boundaries of the jurisdiction of federal law and the laws in the fifty U.S. states and territories.
Federal law
originates with the Constitution, which gives Congress the power to enact statutes for certain limited purposes like regulating interstate commerce. Nearly all statutes have been codified in the United States Code. Many statutes give executive branch agencies the power to create regulations, which are published in the Federal Register and codified into the Code of Federal Regulations. Regulations generally also carry the force of law under the Chevron doctrine. Many lawsuits turn on the meaning of a federal statute or regulation, and judicial interpretations of such meaning carry legal force under the principle of stare decisis.
which was in force at the time of the Revolutionary War. However, the supreme law of the land is the United States Constitution and, under the Constitution's Supremacy Clause, laws enacted by Congress and treaties to which the U.S. is a party. These form the basis for federal laws under the federal constitution in the United States, circumscribing the boundaries of the jurisdiction of federal law and the laws in the fifty U.S. states and territories.Federal law
originates with the Constitution, which gives Congress the power to enact statutes for certain limited purposes like regulating interstate commerce. Nearly all statutes have been codified in the United States Code. Many statutes give executive branch agencies the power to create regulations, which are published in the Federal Register and codified into the Code of Federal Regulations. Regulations generally also carry the force of law under the Chevron doctrine. Many lawsuits turn on the meaning of a federal statute or regulation, and judicial interpretations of such meaning carry legal force under the principle of stare decisis.
State law
The fifty American states are separate sovereigns with their own state constitutions and state governments. They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the federal Senate.
Nearly all states started with the same British common law base, with the notable exception of Louisiana; Louisiana law has always been strongly influenced by the French Napoleonic Code. The passage of time has resulted in enormous diversity in the laws of the states. State courts have expanded the old common law rules in different directions (through their traditional power to make law under the doctrine of stare decisis), and state legislatures have passed various statutes expanding or overriding many judge-made rules
Criminal law
In the arena of criminal law, all states have somewhat similar laws in regard to "higher crimes" (or felonies), such as murder and rape, although penalties for these crimes may vary from state to state.
For public welfare offenses where the state is punishing merely risky (as opposed to injurious) behavior, there is significant diversity across the various states. For example, punishments for drunk driving varied greatly prior to 1990. State laws dealing with drug crimes still vary widely, with some states treating possession of small amounts of drugs as a misdemeanor offense or as a medical issue and others categorizing the same offense as a serious felony.
The fifty American states are separate sovereigns with their own state constitutions and state governments. They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the federal Senate.
Nearly all states started with the same British common law base, with the notable exception of Louisiana; Louisiana law has always been strongly influenced by the French Napoleonic Code. The passage of time has resulted in enormous diversity in the laws of the states. State courts have expanded the old common law rules in different directions (through their traditional power to make law under the doctrine of stare decisis), and state legislatures have passed various statutes expanding or overriding many judge-made rules
Criminal law
In the arena of criminal law, all states have somewhat similar laws in regard to "higher crimes" (or felonies), such as murder and rape, although penalties for these crimes may vary from state to state.
For public welfare offenses where the state is punishing merely risky (as opposed to injurious) behavior, there is significant diversity across the various states. For example, punishments for drunk driving varied greatly prior to 1990. State laws dealing with drug crimes still vary widely, with some states treating possession of small amounts of drugs as a misdemeanor offense or as a medical issue and others categorizing the same offense as a serious felony.