Rule against Trusts of Perpetual Duration is a rule that prohibits noncharitable trusts from lasting beyond the perpetuity period, i.e. a lifetime presently existing plus a period of 21 years. All non-charitable trusts must conform to the rule. A trust that may last beyond that period is void. This is similar to rule against inalienability which prevents property from being rendered incapable of transfer within the perpetuity period.