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In comparative federalism and comparative constitutionalism, reserved powers or residual powers are those powers which are not "enumerated" (written down, assigned). In various federal and decentralized political systems, certain subjects are assigned to either the central (or federal) government or the regional (or state or provincial) government; however it is not possible to list all possible subjects that might be legislated on for all time. Therefore, the framers of major constitutional documents tend to assign all other subjects that may arise after the document is enacted to one of the two orders of government. This is considered a major power in its own right.
In Canada, for example, the reserved powers lie with the federal government; in the United States, the reserved powers lie with the constituent states. 
Reserved powers are distinguished from concurrent powers (which are shared by the federal and constituent governments). Reserved powers are also distinguished from exclusively delegated powers, such as the exclusive federal powers in the United States.
Application around the world
In Australia, despite the centralized nature of the constitution, the High Court adopted the "reserved powers doctrine" which was used until 1920 to interpret the Constitution of Australia such that it should be read to preserve as much autonomy for the states as possible. The doctrine became obsolete with the Engineers' Case; however, the term "residual powers" continues to refer to the powers not explicitly mentioned in the Australian Constitution and, as such, powers reserved to the states as being outside Federal jurisdiction.
In the United Kingdom "reserved matters" are those subjects dealt with solely by the United Kingdom Parliament, and not devolved to the Scottish Parliament, National Assembly for Wales or the Northern Ireland Assembly.
In the United States, the "reserved powers", also called "police powers", are those powers which under the Tenth Amendment to the United States Constitution are reserved to the states or the people. These are sometimes said to be "states' rights".