Rule against perpetuities

The rule against perpetuities is a legal rule in common law property law that states that prohibiting the grant of an estate unless the future interest granting that such estate vests within 21 years after the death of someone alive when the interest was created (lives in being plus twenty one years).

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.