Though considerable research focuses on formal institutions in Congress, scholars have long acknowledged that much of what guides legislative behavior is unwritten. To advance this area, we leverage a tool that allows appropriators to redirect billions of dollars from mandatory programs to discretionary projects. Changes in mandatory program spending-known as “CHIMPs”-show that existing institutions are often maintained by the strategic action of legislators. In the case of CHIMPs, we find their use is largely a response to formal constraints and that they are preserved through avoidance of minimum reform coalitions. This highlights that the legislative process-and budgetary outcomes in particular-cannot be understood without attention to procedures which remain “off the books.”