29 C.F.R. § 4001.3

(1) The PBGC will determine that trades and businesses (whether or not incorporated) are under common control if they are “two or more trades or businesses under common control”, as defined in regulations prescribed under section 414(c) of the Code.

(2) The PBGC will determine that all employees of trades or businesses (whether or not incorporated) which are under common control shall be treated as employed by a single employer, and all such trades and businesses shall be treated as a single employer.

(3) An individual who owns the entire interest in an unincorporated trade or business is treated as his own employer, and a partnership is treated as the employer of each partner who is an employee within the meaning of section 401(c)(1) of the Code.

(1) In connection with any person, a controlled group consists of that person and all other persons under common control with such person.

(2) Persons are under common control if they are members of a “controlled group of corporations”, as defined in regulations prescribed under section 414(b) of the Code, or if they are “two or more trades or businesses under common control”, as defined in regulations prescribed under section 414(c) of the Code.