The rule against perpetuities is a legal rule in common law that prevents people from using legal instruments (usually a deed or a will) to exert control over the ownership of private property for a time long beyond the lives of people living at the time the instrument was written. Specifically, the rule forbids a person from creating future interests (traditionally contingent remainders and executory interests) in property that might feasibly vest beyond 21 years after the lifetimes of those living at the time of creation of the interest, often expressed as a "life in being plus twenty-one years". In essence, the rule prevents a person from putting qualifications and criteria in a deed or a will that would continue to affect the ownership of property long after he or she has died, a concept often referred to as control by the "dead hand" or "mortmain". The practical effect of this is that control can be exercised over no more than two future generations – a will can create restrictions over the future disposal of real property by the testator's son and heir, but not over the further disposal of that property by that son's own heirs. Feasibly, such a third-generation disposal might fall further into the future than a life in being plus twenty-one years – and so terms in a will written to constrain that disposal would be void from the outset.
The basic elements of the rule against perpetuities originated in England in the 17th century and were "crystallized" into a single rule in the 19th century. The rule's classic formulation was given in 1886 by the American legal scholar John Chipman Gray: