Commercial contracts are rarely fully self-contained. UCC 1-303 lets courts read an agreement against two sources of outside context: course of dealing, meaning a sequence of prior conduct between these particular parties establishing a common basis of understanding, and usage of trade, meaning a practice or method regularly observed in a place, vocation, or trade that justifies an expectation it will be observed in the transaction at hand.

A closely related third source, course of performance -- the parties' repeated conduct in performing this very contract -- rounds out the picture. UCC 1-303 arranges the three in a hierarchy: express terms control, but wherever reasonably possible all three should be read as consistent with each other, and where they cannot be reconciled, course of performance outweighs course of dealing, which outweighs usage of trade.

Express contract terms sit at the top. Below that, UCC 1-303(e) ranks course of performance under this contract above course of dealing between these parties in past contracts, which in turn ranks above generalized usage of trade -- each layer supplements and explains the one above it, but does not override it where a genuine conflict cannot be reconciled.

Neither doctrine is satisfied by a single prior interaction or a practice one party merely asserts exists. Course of dealing requires a genuine, repeated pattern between these specific parties sufficient to establish a common basis of understanding. Usage of trade requires the practice be sufficiently regular and widely observed in the relevant place, vocation, or trade that a party in that trade would reasonably expect it to apply -- generally shown through industry standards, trade association materials, or qualified trade-practice testimony.

A proponent builds the record with invoices, past purchase orders, correspondence showing a repeated pattern, or expert and trade-association testimony establishing the claimed practice. An opponent attacks by showing the pattern was too sparse or inconsistent to count as a true course of dealing, that the asserted trade usage is not actually general or well established, or that the interpretation urged cannot be reconciled with an unambiguous express term, which controls regardless.

These doctrines matter most where a contract is silent or genuinely ambiguous on a disputed point -- delivery tolerances, inspection procedures, payment timing. Juricratic treats the strength of course-of-dealing and trade-usage evidence as a dial on the gap-filling and ambiguity-resolution nodes of a claim path, particularly where it interacts with an integration clause purporting to exclude outside evidence or with a battle-of-the-forms dispute over which terms actually govern.