9. Summary of the rule.

At common law, the rule against perpetuities1 may be stated as follows: if an executory devise or other future limitation is to be valid, it must vest if at all within a life or lives in being plus 21 years and a possible period of gestation thereafter. It is not sufficient that it may vest within that period. Such an interest must be good at its creation, and, unless it is created in such terms that it cannot vest after the expiration of that period, it will not be valid and subsequent events cannot make it valid2.

For example, X