Order of performance of reciprocal promises governs who must perform first based on express terms or transaction nature. Order of performance of reciprocal promises: where the contract expressly fixes the sequence, promises must be performed in that order; where no sequence is fixed, performance follows the order which the nature of the transaction requires. For example, a builder ordinarily performs before payment in a fixed-price building contract, whereas delivery of stock may await the buyer's provision of security when the transaction requires it.