The law allows a person preparing a will to have almost complete control over his or her assets after the testator passes on, but there are limits to such power. A person can restrict a property from being sold, or make sure that it is used for a specific purpose. A property can be bequeathed to a family member as long on condition that the person maintains the family business in a specific city, or exercises daily, or places flowers on the deceased's grave every week, or engages in any other behavior the testator desires. This freedom, however, is not without limits. The time limit on this ability is called the rule against perpetuities. The rule is also referred to as the “dead man's hand” statute.

The rule against perpetuities is complex and rarely utilized. At the time of the passing of the testator, the heirs of the estate are locked in. These heirs are referred to as “lives in being.” For the purposes of this rule, if a child is conceived but not yet born at the time of the testator's death, it will be considered a life in being. Once the last living heir named in the will passes away, the restrictions on the property will continue in place as the testator desired for 21 years. The idea is that a testator may control his assets for a full generation after his or her death. The rule is notoriously difficult to apply properly. When it does apply, the conditions on the bequest are abandoned and the gift returns to the residual estate.

What makes this rule so confusing is that, when an individual writes a will, he or she may make gifts to potential children or grandchildren. These children and grandchildren, however, may not be born until years later. If a child has been born at the time the decedent passes away, he or she is subject to the restrictions on the bequest during his or her lifetime. If a grandchild is conceived and born after the decedent's death, however, the child may avoid the restrictions 21 years after the death of the last heir alive at the time of the decedent's death. There is no way to predict when this might occur. The rule is archaic and easily avoided. A knowledgeable attorney can help a person planning his or her estate set up an equitable trust. Similar to a will, a trust may impose conditions on the use of assets, but is not subject to the rule against perpetuities. There are other advantages to a trust, but one of the most important is avoiding this unpredictable and confusing rule.