When Congress is highly polarized and closely divided between the two parties, the majority party will often lack the votes to overcome a Senate filibuster on controversial matters. Majorities have tried to circumvent the filibuster by using the budget reconciliation process--which allows a simple-majority to close debate and move to a final vote--to make major policy on topics ranging from immigration to labor to climate. These efforts sometimes fail on account of the Byrd rule, a rule of legislative procedure that serves as a gatekeeper for the reconciliation process. The most important part of the rule is brief and somewhat cryptic, providing that a provision violates the rule and can be struck from a reconciliation bill if the provision "produces changes in outlays or revenues which are merely incidental to the nonbudgetary components of the provision." This Article excavates and critically examines the body of law that implements this provision of the Byrd rule. First, a key part of the Byrd rule inquiry is an allthings- considered balancing of a provision's budgetary effects as compared to its policy effects. Second, the Senate parliamentarian's office, which administers the Byrd rule, has developed other principles to implement the rule. The office is more likely to find consistent with the rule provisions that, for example, involve direct rather than indirect budgetary impacts, do not impose mandates on private parties, do not target particular entities, and do not concern certain substantive topics. Third, precedent plays a significant role in the Byrd rule inquiry: past decisions carry great weight, though the parliamentarian's office may narrow or depart from precedent in rare but important cases. Recently published materials and original interviews with Senate staffers reveal how these decisional rules shape the operation of the Byrd rule in practice. In addition to its descriptive contribution, this Article also subjects each aspect of Byrd rule doctrine to critical analysis. Some aspects of the doctrine stem naturally from the text of the rule, further a plausible purpose, and are sensible ways of implementing the rule's open-ended language. Other aspects of the doctrine risk being over- or under-inclusive relative to the plausible purposes of the Byrd rule, generate internal tensions in the doctrine, or can give rise to gamesmanship by the parties. But normatively evaluating Byrd rule doctrine on the whole is challenging because there is no present consensus about the rule's purpose, given that doctrine departed from the rule's initial goal of promoting deficit reduction efforts more than two decades ago. Understanding how the Byrd rule works on the ground holds important lessons for the operation of the contemporary Congress, the development of an underappreciated area of law, and the content of public policy. [ABSTRACT FROM AUTHOR]