A multistate business can have the same dollar of income included in the apportionable base of several states at once, and the result is constitutional. That outcome follows from a framework in which each state may tax the portion of a unitary enterprise's income fairly attributable to activity within its borders, using a formula it selects, without any requirement that the formulas be consistent across states. This article works through the constitutional test that governs state taxation of interstate commerce, the unitary business principle that determines what income is apportionable at all, the formula rules and the limited constraints on them, and the combined reporting regimes that treat affiliated corporations as one taxpayer. It closes with the nexus revolution that replaced physical presence with an economic standard.