Combining previous work on market formation and regulation with a case study of the emerging legal cannabis markets in the United States, we develop the argument that interrelations to other markets contribute significantly to constitute the social systems of regulated markets. Specifically, market interrelations enacted during legitimation and regulation influence who becomes involved in the market formation process and direct attention to specific issues in that process. After successfully (re)regulating a market, new interrelations are enacted via practices borrowed from historic, parallel and auxiliary markets, and via material influences based on complementarity and substitutability. While these multiple interrelations to other markets complicate market delineation, they are also a historical precondition for it.