  Suppose, for example, that X, under his will, left a sum of money to Y, on condition that Y had attained the age of 21. It may be that the correct way of understanding the provision, under the rules of the legal system, is that only if Y had attained 21 when X died does he have a right to the money. But it may be that the correct way of understanding it as saying that Y, even if he has not attained 21 when X dies, acquires a right to the money, but it is to be paid only when he is 21. One practical difference is that in the latter case the right can pass to Y’s successor in title if Y, having survived X, nevertheless dies before he is 21. In the latter case, lawyers describe the right as ‘vested.’ There can be many complex legal rules relating to this type of situation, and they vary greatly from jurisdiction to jurisdiction. Reference should be made to textbooks, primarily on testamentary succession, in the jurisdiction.