Definition
Dual sovereignty is the idea at the heart of American federalism: the national government and the state governments are each genuinely sovereign, not as a hierarchy of master and servant but as two distinct sources of authority that draw power directly from the people. Each level governs within its own constitutional sphere, and within that sphere it acts in its own name.
The federal government holds enumerated powers delegated by the Constitution, while the states retain a broad reserve of authority over matters such as criminal law, family law, property, and education. The Tenth Amendment expresses the residue: powers not delegated to the United States, nor prohibited to the states, are reserved to the states or to the people.
Why it matters
How it works
Because authority is divided rather than nested, conflicts are resolved by subject matter and by the Supremacy Clause rather than by simple chain of command. Where the federal government acts within its enumerated powers, valid federal law prevails over conflicting state law. Where it does not, the states retain their reserved authority. Courts police the boundary, deciding when an exercise of federal power exceeds its constitutional grant and when state action intrudes on a federal domain.