The leading authorities to know, and how the doctrine developed over time. A promise is enforceable as a contract only if supported by consideration, something of value (a benefit to the promisor or a detriment to the promisee) given in exchange for the promise. Past consideration is no consideration; consideration must move from the promisee.
The pre-existing duty rule, a promise to do what you are already contractually bound to do is not good consideration.
The classical statement of consideration, benefit and detriment formulation that students must be able to recite.
Part-payment of a debt is not good consideration for a promise to discharge the balance, the rule promissory estoppel later qualifies.
Softened Stilk by recognising practical benefit as sufficient consideration in variation cases, pivotal for any modern consideration question.