Royal Lives Clause

A Royal lives clause is a contract clause which provides that a certain right must be exercised within (usually) the lifetime plus 21 years of the last living descendant of a British Monarch who happens to be alive at the time when the contract is made.
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A clause inserted in many trusts to determine their maximum durations. A prominent person - often the late British monarch King George VI - is selected, and termination of the trust is determined by the death of the last lineal descendant of that person living at the time the trust takes effect, plus twenty one years.