Measuring the cost of legislation or even projecting the course of the federal budget requires defining a budget baseline—a starting point capturing the current state of the budget. Budget baselines underlie most measures employed in federal budget debates and enforcement rules. Yet, despite their widespread use, budget baselines engender considerable confusion and abuse. For instance, when legislators enact temporary tax breaks, the breaks are officially estimated to cost far less than they likely will because of a loophole in federal budget baseline rules. Then, later efforts to extend the tax cuts are counted as increasing deficits when, in fact, by more reasonable metrics, they do nothing of the sort and might even reduce deficits. In r...