Rule Against Perpetuities
The Rule Against Perpetuities is a legal doctrine developed in English common law to limit the right of ownership of private property with respect to the use or sale of property in the distant future ("perpetuities"). The theory is that future uses of land should be dictated by free enterprise, not by the desires of an owner in the distant past.
The Rule accomplishes its purpose by prohibiting any future ownership or control of land that does not "vest" or become effective until 21 years after the death of a living person. This Rule allows a parent to grant his property to his future children only upon their reaching adulthood. Planning further in the future for the control of land is blocked by this Rule.
Another way to understand the Rule is this: no one can control any property longer than 21 years after everyone who is alive today has died.