This chapter highlights important conceptual differences between swing-contract (SC) markets and current US RTO/ISO-managed wholesale power markets. Current ISO markets are based on the fiction that the essential product being bought and sold in these markets is energy measured in megawatt-hours. The chapter argues that the classic arguments in favor of uniform pricing in competitive commodity markets do not apply to SC markets because the product transacted in SC markets is not a commodity, i.e. not an asset whose units are perfect substitutes for each other. The Law of One Price is a general precept that follows immediately from the definition of a commodity. The chapter compares basic attributes and optimization formulations for SC markets and current US RTO/ISO-managed wholesale power markets, focusing on day-ahead markets (DAMs) for concreteness. Basic advantages of the SC DAM design are highlighted.