20 C.F.R. § 404.1005

You must be an employee for your work to be covered as employment for social security purposes. You are an employee if you are—

(a) A corporation officer as described in § 404.1006;

(b) A common-law employee as described in § 404.1007 (unless you are, after December 31, 1982, a qualified real estate agent or direct seller as described in § 404.1069); or

(c) An agent-driver or commission-driver, a full-time life insurance salesman, a home worker, or a traveling or city salesman as described in § 404.1008.