26 CFR §1.414(c)-6

1. (a)General rule. Except as provided in paragraph (b), (c), (e), (f), or (g) of this section, the provisions of § 1.414(b)-1 and §§ 1.414(c)-1 through 1.414(c)-4 apply for plan years beginning after September 2, 1974.
2. (b)Existing plans. In the case of a plan in existence on January 1, 1974, unless paragraph (c) of this section applies, the provisions of “§ 1.414 (b)-1 and §§ 1.414(c)-1 through 1.414(c)-4 shall apply for plan years beginning after December 31, 1975. For definition of the term “existing plan”, see § 1.410(a)-2(c).
3. (c)Existing plans electing new provisions. In the case of a plan in existence on January 1, 1974, for which the plan administrator makes an election under § 1.410 (a)-2(d), the provisions of § 1.414(b)-1 and §§ 1.414 (c)-1 through 1.414(c)-4 shall apply to the plan years elected under § 1.410 (a)-2 (d).
4. (d)Application. For purposes of the Employee Retirement Income Security Act of 1974, the provisions of § 1.414(b)-1 and §§ 1.414(c)-1 through 1.414(c)-4 do not apply for any period of time before the plan years described in paragraph (a), (b), or (c) of this section, whichever is applicable.
5. (e)Special rule. Notwithstanding paragraph (a), (b), or (c) of this section, § 1.414(c)-3 (f) is effective April 1, 1988.
6. (f)Transitional rule
   1. (1)In general. The amendments made by T.D. 8179 apply to the plan years or period described in paragraphs (a), (b), or (c) of this section, whichever is applicable.
   2. (2)Exception. In the case of a plan year or period beginning before March 2, 1988, if an organization—
      1. (i)Is a member of a brother-sister group of trades or businesses under common control under § 11.414(c)-2(c), as in effect before removal by T.D. 8179 (“old group”), for such plan year or period, and
      2. (ii)Is not such a member for such plan year or period because of the amendments made by such Treasury decision,
7. (g)Special rule. Notwithstanding paragraph (a), (b), or (c) of this section, § 1.414(c)-2(b)(1) applies to plan years beginning on or after January 1, 2025.