Under the rules and customs of the Senate, a quorum is always assumed present unless a quorum call explicitly demonstrates otherwise. A senator may request a quorum call by "suggesting the absence of a quorum"; a clerk then calls the roll of the Senate and notes which members are present. In practice, senators rarely request quorum calls to establish the presence of a quorum. Instead, quorum calls are generally used to temporarily delay proceedings; usually such delays are used while waiting for a senator to reach the floor to speak or to give leaders time to negotiate. Once the need for a delay has ended, a senator may request unanimous consent to rescind the quorum call. Debate, like most other matters governing the internal functioning of the Senate, is governed by internal rules adopted by the Senate. During debate, senators may only speak if called upon by the presiding officer, but the presiding officer is required to recognize the first senator who rises to speak. Thus, the presiding officer has little control over the course of debate. Customarily, the majority leader and minority leader are accorded priority during debates even if another senator rises first. All speeches must be addressed to the presiding officer, who is addressed as "Mr. President" or "Madam President", and not to another member; other Members must be referred to in the third person. In most cases, senators do not refer to each other by name, but by state or position, using forms such as "the senior senator from Virginia", "the gentleman from California", or "my distinguished friend the chairman of the Judiciary Committee". Senators address the Senate standing next to their desk. Apart from rules governing civility, there are few restrictions on the content of speeches; there is no requirement that speeches pertain to the matter before the Senate. The rules of the Senate provide that no senator may make more than two speeches on a motion or bill on the same legislative day. A legislative day begins when the Senate convenes and ends with adjournment; hence, it does not necessarily coincide with the calendar day. The length of these speeches is not limited by the rules; thus, in most cases, senators may speak for as long as they please. Often, the Senate adopts unanimous consent agreements imposing time limits. In other cases (for example, for the budget process), limits are imposed by statute. However, the right to unlimited debate is generally preserved. Within the United States, the Senate is sometimes referred to as "world's greatest deliberative body". The filibuster is a tactic used to defeat bills and motions by prolonging debate indefinitely.