fee simple determinable:
(18c)

1. An estate that will automatically end and revert to the grantor if some specified event occurs (e. g., “to Albert and his heirs while the property is used for charitable purposes”); an estate in fee simple subject to a special limitation.  *  The future interest retained by the grantor is called a possibility of reverter. — Abbr. FSD. — aka determinable fee; qualified fee; fee simple subject to common-law limitation; fee simple subject to special limitation; fee simple subject to special interest; base fee; estate on limitation. [1]

     Excerpt from William B. Stoebuck & Dale A. Whitman’s The Law of Property (3d ed. 2000):

     “In theory, it should be easy to determine whether an instrument creates a fee simple determinable or a fee simple subject to a condition subsequent. If the instrument includes a special limitation (introduced by words such as ‘so long as’ or ‘until’) it creates a fee simple determinable, whether or not it also includes an express reverter clause. If the instrument includes an express condition or proviso (‘on condition that’ or ‘provided that’) and an express right to re-enter for breach of the stated condition, it creates a fee simple subject to a condition subsequent. But deeds and wills often fail to employ the appropriate words to create one of the two types of defeasible estate or the others. Instead deeds and wills often contain a confusing mixture of words appropriate for creation of both types of defeasible estate.” [2]

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