 Contracts, in other words, arise out of an exchange of promises where each party’s promise triggers the other’s, in what Oliver Wendell Holmes (1881 [1991: 293–294]) calls “reciprocal conventional inducement”. In exceptional circumstances, a gratuitous promise may be enforced in the absence of consideration. Certain promises (especially concerning contract modifications) made among merchants are enforceable without consideration, as are promises to make charitable subscriptions (R2 §90(2); Uniform Commercial Code [“UCC”] § 2-209). More generally, the doctrine of promissory estoppel enforces promises that are detrimentally relied on when considerations of justice demand it (R2 §90). But promissory estoppel is in practice narrowly construed and rarely invoked by courts (for a leading case applying the rule in the context of pre-contractual bargaining, see Hoffman v. Red Owl Stores, Inc. [1965], and for a survey of the law that shows Hoffman’s exceptional status, see Schwartz and Scott [2007]).