(a) Designation of vesting computation periods. Except as provided in paragraph (b) of this section, a plan may designate any 12-consecutive-month period as the vesting computation period. The period so designated must apply equally to all participants. This requirement may be satisfied even though the actual 12-consecutive-month periods are not the same for all employees (e.g., if the designated vesting computation period is the 12-consecutive-month period beginning on an employee's employment commencement date and anniversaries of that date). The plan is prohibited, however, from using any period that would result in artificial postponement of vesting credit, such as a period meassured by anniversaries of the date four months following the employment commencement date.

(b) Plans with 3-year 100 percent vesting. For rules regarding when a participant has a nonforfeitable right to his accrued benefit, see section 202(a)(1)(B)(i) of the Act and section 410(a)(1)(B)(i) of the Code and regulations issued thereunder.

(c) Amendments to change the vesting computation period. (1) A plan may be amended to change the vesting computation period to a different 12-consecutive-month period provided that as a result of such change no employee's vested percentage of the accrued benefit derived from employer contributions is less on any date after such change than such vested percentage would be in the absence of such change. A plan amendment changing the vesting computation period shall be deemed to comply with the requirements of this subparagraph if the first vesting computation period established under such amendment begins before the last day of the preceding vesting computation period and an employee who is credited with 1,000 hours of service in both the vesting computation period under the plan before the amendment and the first vesting computation period under the plan as amended is credited with 2 years of service for those vesting computation periods. For example, a plan which has been using a calendar year vesting computation period is amended to provide for a July 1-June 30 vesting computation period starting in 1977. Employees who complete more than 1,000 hours of service in both of the 12-month periods extending from January 1, 1977 to December 31, 1977 and from July 1, 1977 to June 30, 1978 are advanced two years on the plan's vesting schedule. The plan is deemed to meet the requirements of this subparagraph.

(2) For additional requirements pertaining to changes in the vesting schedule, see section 203(c)(1) of the Act and section 411(a)(10) of the Code and the regulations issued thereunder.

(d) Service preceding a break in service. For purposes of applying section 203(b)(3)(D) of the Act and section 411(a)(6)(D) of the Code, (relating to counting years of service before a break in service for vesting purposes), the computation periods used by the plan in computing years of service before such break must be the vesting computation periods. (For application of the break in service rules, see section 203(b)(3)(D) and section 411(a)(6)(D) of the Code and regulations issued thereunder.)