Citizenship of the United States
A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.
Identity documents in the United States
For citizens who acquire United States citizenship not by virtue of being born in the United States, the federal government issues a Certificate of U.S. Citizenship or Certificate of Naturalization, which are documents that function similarly to a birth certificate. These two documents, along with a U.S. passport, are by law one of the few primary documents for proving U.S. citizenship. These certificates are normally not carried on a day-to-day basis; instead, they are used to procure other documents, such as a passport or driver's license, which are then carried and used as a primary means of identification.