Dating back to 17th-century England, the rule against perpetuities is a legal concept that places restrictions on the control and transfer of property in certain circumstances. Specifically, it prevents someone from structuring their will or creating a trust in a manner that allows the property (usually real estate) to remain in the family "forever."
As written, the rule against perpetuities ensures that someone actually takes ownership of the property willed or entrusted to them within a designated period. This period, as stated under the rule, is 21 years after the death of a person who was alive at the time that the will or trust was created.
Any ambiguity regarding whether the transfer of property could reasonably occur within the these parameters renders the will or trust void.
Due to its complexity, this common law rule has been changed or abandoned in some states.