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The transfer of most or all sovereignty from two or more states to form a single state. A political union may be a unitary state; that is, all constituent states cease to have legal existence (except maybe as administrative divisions) and they are subject to the same law. Alternatively, a political union may be federal, in which the new state is given certain responsibilities and remaining powers stay with the previously independent states. Political unions generally have one currency and have no trade barriers between constituent states.