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An interest in real property that has the potential to last forever, provided a specific contingency does not occur.
For example, a grantee might be given an estate in blackacre, "provided the land is not used for illegal purposes."
This type of fee is also known as a conditional, determinable, or qualified fee.
BASE FEE, English law. A tenure in fee at the will of the lord. This was distinguished from socage free tenure. See Co. Litt. 1, 18.