See: Defeasible Title. An estate that ends for one of two reasons. There can be no more heirs or there may be special limitations which come to pass in the form of stated conditions subsequent, or executory limitations.
It is an elementary principle of property law that the words 'fee simple' followed by a condition or special limitation create a defeasible fee. Hersey v. Purington, 96 Me. 166, 51 A. 865 (1902); C. Moynihan, Introduction to the Law of Real Property 35 (1962). The fee simple determinable and the fee simple subject to a condition subsequent. A fee simple determinable is a fee subject to a special limitation. The principal difference between a fee simple determinable and a fee simple subject to condition subsequent is that the former expires automatically on the happening or non-happening of a specified event, while the latter is subject to a power in the grantor, called the right of re-entry, to terminate the estate on the happening of a specified event such as, here, a breach of the condition. Moynihan, supra, at 36.
While no precise and technical words are required to create a conditional fee, terms such as 'upon condition ...