Perpetuity rule limits restraints on alienation by forbidding dispositions that suspend transfer beyond lives in being plus term. Perpetuity denotes a legal limitation preventing or suspending the power of alienation beyond lives in being plus an additional term, designed to prevent removing property from commerce for longer than the permitted survival period; the rule accounts for posthumous births by extending the measuring period by the term of gestation and functions to invalidate or constrain dispositions creating prolonged restraints on alienation.