This paper constructs a unified theory of the location of transactions and the boundaries of firms. It proposes that systems of production can be viewed as networks of tasks. Transactions, defined as mutually agreed-upon transfers with compensation, are located within the task network and serve to separate one set of tasks from another. Placing a transaction in a particular location in turn requires work to define, count (or measure), and pay for the transacted objects. The costs of this work (labeled mundane transaction costs) are generally low at module boundaries and high in their interiors. Several novel implications arise from this work. Among these: Modularizations create new module boundaries, hence new transaction locations where entry and competition can arise. Areas in the task network where transfers are dense and complex should not be modularized. Instead these areas should be located in transaction-free zones so that the costs of transacting do not overburden the system. The boundaries of transaction-free zones constitute breakpoints where firms and industries may split apart.