<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> <P> Social Security numbers were first issued by the Social Security Administration in November 1935 as part of the New Deal Social Security program . Within three months, 25 million numbers were issued . </P> <P> On November 24, 1936, 1,074 of the nation's 45,000 post offices were designated "typing centers" to type up Social Security cards that were then sent to Washington, D.C. On December 1, 1936, as part of the publicity campaign for the new program, Joseph L. Fay of the Social Security Administration selected a record from the top of the first stack of 1,000 records and announced that the first Social Security number in history was assigned to John David Sweeney, Jr., of New Rochelle, New York . </P>

When did it become mandatory to have a social security number