Non-tax legal rules regulating the workplace, the financial sector, real property, and many other areas affect the ability of governments to collect revenues and provide public goods. Yet tax-collection considerations rarely enter into economic analyses of non-tax legal rules. Usually, tax-collection concerns are shunted aside to separate studies (and separate law-school courses) rather than being integrated into debates in non-tax spheres. This separation between non-tax legal rules and tax-collection considerations bears significant negative consequences for the ability of law and economics to generate descriptively accurate and normatively attractive accounts of important non-tax legal questions. This Article takes a step toward remedying that oversight. We present an analytic framework for understanding the interaction between non-tax legal rules and tax collection. This framework—which we call the Legal Envelope Theorem—demonstrates that legal rules should systematically deviate from simple notions of efficiency to take stock of tax effects. We then provide a series of examples applying the Legal Envelope Theorem, illustrating how the non-tax legal system ought to be (and, on occasion, actually is) designed with tax effects in mind. These examples range from parental-leave mandates to bank capital requirements to centuries-old property and contract rules regularly taught in introductory law-school courses. We illustrate how a framework that is attentive to tax-collection considerations can enhance the government’s capacity to redistribute resources and address wealth inequality.