Condition subsequent

A condition subsequent is an event or state of affairs that brings an end to something else. A condition subsequent is often used in a legal context as a marker bringing an end to one's legal rights or duties. A condition subsequent may be either an event or a state of affairs that must either (1) occur or (2) fail to continue to occur.

A condition subsequent may be either an event or a state of affairs that must either (1) occur or (2) fail to continue to occur.

An example of the first, a condition that must occur to bring an end to something else:

An example of the second, a condition that must fail to occur to bring an end to something else:

In both cases, running out of fuel is a condition subsequent to the continuance of the fire.

A condition subsequent is noted for its common use in the law.

In contract law:
A contract may be frustrated on the occurrence of a condition subsequent: in a contract to provide a music hall for a musical performance, the burning down of the music hall may frustrate the contract and automatically bring it to an end. Taylor v Caldwell 3 B. & S. 826, 122 Eng. Rep. 309 (1863)

In property law:

Main Article: Fee simple subject to condition subsequent

A right in land may be cut off by a condition subsequent. When land rights are subject to a condition subsequent, this creates a defeasable fee called a Fee simple subject to condition subsequent.

In such a fee, the future interest is called a "right of reentry" or "right of entry." There, the fee simple subject to condition subsequent does not end automatically upon the happening of the condition, but if the specified future event occurs, the grantor has a right to retake his property (as opposed to it reverting to him automatically). Again, the right of entry is not automatic, but rather must be exercised to terminate the fee simple subject to condition subsequent. To exercise right of entry, the holder must take substantial steps to recover possession and title, for example, by filing a lawsuit.

One of the languages used to create a fee simple subject to condition subsequent and a right of entry is "to A, but if A sells alcohol on the land, then grantor has the right of reentry."

...
Wikipedia