A notoriously complicated rule of property law, abolished in a number of American jurisdictions and altogether abolished in England. The Rule has plagued law students since the Seventeenth Century.

The aim of the Rule to ensure that a future property interest vest or fail to vest within 21 years of the death of a "validating life." This is merely another way of saying that the purpose of the Rule is to prevent you from writing a will that leaves some piece of property "to my son, then to his child, then to his child," and so on. Legal thinkers viewed this as counter-utilitarian, i.e. that it goes against the best-use principle (land should always be afforded its best use to promote economic growth)and allows a long-dead ancestor to exert control from beyond the grave in deciding who owns what land.