In addition to being classified as to their quantity, estates are also classified as to their quality. By the quality of the estate, is meant its certainty of continuance. Estates as to their quality are first divided into absolute estates and conditional estates. Any estate, whatever its quantity, may be either absolute or conditional. An absolute estate is one which is certain, that is, an estate which is certain to continue for the full period for which it is granted. The termination of an absolute estate may be determined by an event whose exact date is uncertain, as for example, the death of the tenant.

A conditional estate, or as it is variously called, a base, determinable, or qualified estate, is one which involves the element of uncertainty. It is an estate whose commencement, continuation, or termination is dependant upon the happening or not happening of some event. Expressed in another way, an estate of this character is one which may never come into existence, or which may be terminated before the expiration of the full period for which it was granted. The various classes of conditional base determinable qualified estate are as follows: fee-conditionals; estates on condition, such estates being divided into estates on condition precedent, and estates on conditions subsequent; estates upon limitation; and conditional limitations.