What is the primary condition that determines the validity of an interest under the Rule Against Perpetuities?

The primary condition that determines the validity of an interest under the Rule Against Perpetuities is that the interest must vest within 21 years after the death of a life in being at the creation of the interest. This means that any future interest or conditional interest must either become possessory or be capable of becoming possessory within this specified time frame.

The rationale behind this rule is to prevent interests in property from being indefinitely delayed, which could create uncertainty about ownership and hinder the transferability of property. The "life in being" refers to a person who is alive at the time the interest is created, and the 21-year period starts from the moment that person's death. This allows for a clear timeframe in which the interest must be resolved, fostering legal certainty in property rights.

Understanding this principle is crucial for evaluating future interests in property and ensuring that any limitations placed on transferability do not violate the established temporal limits set forth by the Rule Against Perpetuities.