Treas. Reg. § 1.414(b)-1

(a) Defintion of controlled group of corporations. For purposes of this section, the term “controlled group of corporations” has the same meaning as is assigned to the term in section 1563(a) and the regulations thereunder, except that (1) the term “controlled group of corporations” shall not include an “insurance group” described in section 1563(a)(4), and (2) section 1563(e)(3)(C) (relating to stock owned by certain employees’ trusts) shall not apply. For purposes of this section, the term “members of a controlled group” means two or more corporations connected through stock ownership described in section 1563(a) (1), (2), or (3), whether or not such corporations are “component members of a controlled group” within the meaning of section 1563(b). Two or more corporations are members of a controlled group at any time such corporations meet the requirements of section 1563(a) (as modified by this paragraph). For purposes of this section, if a corporation is a member of more than one controlled group of corporations, such corporation shall be treated as a member of each controlled group.

(b) Single plan adopted by two or more members. If two or more members of a controlled group of corporations adopt a single plan for a plan year, then the minimum funding standard provided in section 412, the tax imposed by section 4971, and the applicable limitations provided by section 404(a) shall be determined as if such members were a single employer. In such a case, the amount of such items and the allocable portion attributable to each member shall be determined in the manner provided in regulations under sections 412, 4971, and 404(a).

(c) Cross reference. For rules relating to the application of sections 401, 408(k), 410, 411, 415, and 416 with respect to two or more trades or businesses which are under common control, see section 414(c) and the regulations thereunder.