Restraints on the legal power of alienation which arise by acts of the parties can be classified into three categories: disabling restraints, forfeiture restraints and promissory restraints. A disabling restraint exists when the property involved is under a direction that it shall not be alienated. A forfeiture restraint exists when the property involved will be forfeited upon alienation of the property by the conveyee. A promissory restraint exists when the conveyee has promised not to alienate the property; it may arise out of a covenant either in the conveyance itself or in a separate contract.
Herbert A. Bernhard,
The Minority Doctrine Concerning Direct Restraints on Alienation,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol57/iss8/3