<P> In the United States, a Social Security number (SSN) is a nine - digit number issued to U.S. citizens, permanent residents, and temporary (working) residents under section 205 (c) (2) of the Social Security Act, codified as 42 U.S.C. § 405 (c) (2). The number is issued to an individual by the Social Security Administration, an independent agency of the United States government . Although its primary purpose is to track individuals for Social Security purposes, the Social Security number has become a de facto national identification number for taxation and other purposes . </P> <P> A Social Security number may be obtained by applying on Form SS - 5, Application for A Social Security Number Card . </P>

What is the purpose of the social security number