The creator of a trust of any complexity or length of duration usually attempts to create future contingent interests under the trust, or interests to take effect after the termination of the trust. In making provisions for such contingent future interests, the settlor must be careful to respect the Rule Against Remoteness of vesting, if he would have his trust secure against attack. Thus, if S wishes to create a trust by his will in favor of his children and grandchildren, providing for grandchildren who may be born after S's death, it is obvious that he desires to provide for future contingent interests for such possible after-born grand-children. Their interests cannot be vested until they are born. This settlor must therefore be careful to provide for the vesting of such future contingent interests at a date not more remote than the end of a life or lives in being at the time of S's death, plus twenty-one years thereafter. This is exactly what took place in Mercer v. Mercer, but the court saw fit by some strange reasoning to declare it void because it was in violation of the Rule Against Perpetuities.
Reuben M. Payne, The Rule Against Perpetuities and Its Application toa Private Trust, 1(2) Clev.-Marshall L. Rev. 59 (1952)