A rule developed by the common law to enable a court to declare void any future or postponed interest in property that might possibly vest (i.e. become enjoyable as of right) outside the perpetuity period. This comprises (1) the lifetimes of persons mentioned (or mentioned by implication) in the disposition who are alive at that time and whose existence governs the timing of the vesting of the future interest (the relevant lives in being), plus (2) 21 years, plus (3) (in the case of a posthumous beneficiary) any actual period of gestation. The period runs from the date of execution of the deed or, if the disposition is contained in a will, from the death of the testator. The purpose of the rule is to prevent land being tied up for an indefinite period, which would hinder its ultimate disposal.
The common law rules relating to perpetuities have been supplemented (though not replaced) by the Perpetuities and Accumulations Act 1964. When a disposition would otherwise be void under the common law rules, the Act allows a period in which one can wait and see whether the interest will in fact vest within the perpetuity period; only if it becomes clear that it cannot do so will the disposition be void. For these purposes, the perpetuity period is varied in that the Act provides a new list of lives in being (the statutory lives in being). The Act also provides for a fixed perpetuity period unrelated to lives.