In law, one who resides in a country without becoming naturalized, retaining instead the citizenship of another country. The laws of most nations have long afforded aliens certain minimum standards of civilized treatment but have also restricted their employment and ownership of property. Under U.S. law, all aliens have had to register since 1940. Registration cards (green cards) entitle them to obtain employment. Like citizens, aliens are protected by the U.S. Constitution, including the Bill of Rights and the due-process clause of the 14th Amendment. They remain subject to limitations under local laws, and residence in the U.S. is not a right but a privilege granted by Congress.