Rule against perpetuities

The rule against perpetuities is a legal rule in common law property law which prohibits the grant of an estate where the persons entitled to inherit a future interest cannot be determined with absolute certainty within 21 years after the death of someone alive when the interest was created. This is often expressed as lives in being plus twenty one years. When determining the time in which an estate will vest a person is considered fertile from the time of birth to the time of death.

Quote

• The common law Rule Against Perpetuities (modified by statute in some states) provides that no interest is valid unless it must vest within 21 years after lives in being when the interest was created. The rule is something of a misnomer. It does not limit the duration of a condition of a bequest, but rather limits the testator's power to earmark gifts for remote descendants.

— Richard Posner Economic Analysis of the Law 2nd ed. (1977), sec. 18.7 at page 394.