Some people put a clause in their will stating that anyone receiving property under the will must survive for thirty days (or forty-five or sixty) after the death of the person who made the will. This is so if two people die in the same accident, there will not be two probates and the property will not go to the other party's heirs.
Example: Fred and Wilma were married and each had children by previous marriages. They did not have survivorship clauses in their wills, and they were in an airplane crash and died. Fred's children hired several expert witnesses and a large law firm to prove that at the time of the crash Fred lived for a few minutes longer than Wilma. That way, when Wilma died first, all of her property went to Fred. When he died a few minutes later, all of Fred and Wilma's property went to his children. Wilma's children got nothing.