26 CFR 301.7701-11: -11 Social security number.

For purposes of this chapter, the term social security number means the taxpayer identifying number of an individual or estate which is assigned pursuant to section 6011(b) or corresponding provisions of prior law, or pursuant to section 6109, and in which nine digits are separated by hyphens as follows: 000-00-0000. Such term does not include a number with a letter as a suffix which is used to identify an auxiliary beneficiary under the social security program. The terms “account number” and “social security number” refer to the same number.