Most people believe that, in today's partisan environment, the filibuster prevents the Senate from acting on all but the least controversial matters. But this is not exactly correct. In fact, the Senate since the 1970s has created a series of special rules--described by Molly Reynolds as "majoritarian exceptions"--that limit debate on a wide range of measures on the Senate floor.

The details of these exemptions might sound arcane and technical, but in practice they have enabled the Senate to act even when...