The Rule against Perpetuities is one of the most complicated rules in property law and wills and trusts. In general, the rule states that a gift of a will (or a trust) will not be enforced if the subject of the gift is to be given to a person who can not be known now, and will not be able to be known, by the end of 21 years after the death of everyone who was alive (including children in their mother's uterus but not yet born) at the time the gift was created. 