In the House, the presiding officer is either the Speaker, or his designee, known as "The Speaker pro tempore," meaning the Speaker for a time (temporary Speaker.) A Speaker pro tem is named daily to substitute for the Speaker when needed, and is always a Member of the majority party. For years, the practice in the House has been for the Speaker to only preside over the House in the first few convening moments of each daily session and on ceremonial and politically significant occasions. As the job of Speaker has expanded over the years and his list of duties grown, the role of presiding over House proceedings has been increasingly delegated to others. So, when Members use the terms "Mr. or Madame Speaker" they are most likely addressing the Speaker pro tem, not the Speaker of the House.

If you watch the House floor regularly, you will also sometimes hear the chair addressed as "Mr. Chairman/Madame Chairman." This occurs when the House meets in the form of a committee for certain legislative proceedings. When the House is sitting as the Committee of the Whole, the speaker pro tempore leaves and a new presiding officer is appointed to act as chair of the Committee.

In the Senate, the presiding officer is named by the Constitution as the Vice-President of the United States, who shall serve as the "President of the Senate." In his absence, the Constitution calls for a "President pro tempore" to be selected (or temporary President). The role of President pro tem is filled by the most senior Senator of the majority party.

The Vice-President has countless other duties today and is rarely in the Senate except for convening new sessions of Congress, for ceremonial occasions, and when a tie vote is expected and his presence may be needed to break the tie. And the President pro tem, being so senior, is also a committee chairman and unable to commit large amounts of time to presiding. In his absence, a rotation of junior Senators of the majority party preside. However, regardless of who is in the chair, "Mr. President" remains the correct form of address in the Senate.

The reason you hear Members use "Mr. Speaker/Mr. President" so frequently is because the rules of procedure in both chambers require Members to address their remarks to the Chair, and not directly to one another. Although the rule is not always enforced, its purpose is to keep all references in the third person, rather than in the first -- a rhetorical device to keep floor speech depersonalized. Using the phrase, "Mr. Speaker, will the gentleman from Missouri respond to a question," rather than saying "Joe, answer my question!" is meant to remind Members, regardless of the heat of debate, to remain respectful toward one another's position as a representative of thousands of American citizens.