Unilateral and Bilateral Contracts Compared

Uniltaeral Contract
Anders makes an offer to enter into a unilateral contract with Daniel, promising to pay him $1,000 for his watch. If Daniel gives Anders his watch in exchange for his promise to pay $1,000 (thereby accepting the offer). Anders' promise to pay $1,000 is enforceable under the benefit/detriment theory. That is, Daniel, the promisee, suffered a legal detriment (i.e. she gave up possession of his watch), and Anders, the promisor, received a legal benefit (i.e. he obtained possession of the watch). Further, the detriment and benefit were given in exchange for Anders' promise to pay $1,000. Thus, the parties have entered into a valid, enforceable contract with a promissory offer, an acceptance by conduct, and consideration to enforce Anders' promise to pay.

Bilateral Contract
Assume that instead of immediately handing the watch to Anders in response to his offer to pay $1,000 for his watch, Daniel promises to give Anders his watch at a later point.

Anders' promise to pay is enforceable, for Daniel suffered legal detriment (he promised to give up possession of the watch), and Anders obtained a legal benefit (he was promised possession of the watch) in exchange for his promise of the $1,000 payment.

Note that Daniel's promise to give up the watch is also enforceable. Anders suffered a legal detriment, payment of $1,000. Daniel obtained a legal benefit (receipt of Anders' promise to pay), in exchange for Daniel's promise to tender the watch. As such, the parties have entered into a valid, enforceable bilateral contract, with a promissory offer, a promissory acceptance,and consideration to support the executory promises.