Senate Rule XXV and party conference rules address committee assignments. Senate Rule XXV, paragraphs 2 and 3 establish categories of committees, popularly referred to as "A," "B," and "C," that condition assignment rules.

Number of Assignments: Senate Rule XXV, paragraph 4, places restrictions on committee membership based on these categories.

• Each Senator shall serve on two committees, and no more than two, in Class A.
• Each Senator may serve on one committee, but no more than one, in Class B.
• Each Senator may serve on one or more committees in Class C.

Limitations on Assignments:

• Super "A" or "Big Four" Committees: Democratic Members are prohibited from serving on more than one among the Committees on Appropriations, Armed Services, and Finance pursuant to party rules. Republican Members are prohibited from serving on more than one among the Committees on Appropriations, Armed Services, Finance, and Foreign Relations pursuant to party rules.
• Republican Members from the same state are prohibited from serving on the same committee by Republican Conference rule. Democrats adhere to the same prohibition, but by tradition.
• Intelligence Committee membership should include two Members each from the Committees on Appropriations, Armed Services, Judiciary, and Foreign Relations.
• Chair and ranking Member of the Armed Services Committee shall serve as ex-officio non-voting members of the Intelligence Committee.

Exceptions to the rule restrictions, which are not uncommon, are called waivers or grandfathers and are recommended by the pertinent party conference and then officially authorized through Senate approval of a resolution affecting one or more Senators.

Limitations on Chairmanships: Republican Conference rules address limitations on committee leadership positions for Republicans whether or not they are in the majority. Democratic Conference rules may also address these limitations; however, their rules are not publicly available.

• A chair/ranking Member of an "A" committee may not serve as chair or ranking Member of any other committee, except the chair/ranking Member of the Finance Committee may chair the Joint Committee on Taxation.
• A chair/ranking Member of an "A" committee may not serve as chair/ranking Member of any subcommittees. Appropriations subcommittee chairmanships are exempt.
• A chair/ranking Member of a "non-A" committee may not serve as chair/ranking Member of any other committee, except the chair/ranking Member of the Rules and Administration Committee may chair the Joint Printing or Joint Library Committee.
• A chair/ranking Member of a "non-A" committee, excluding the Ethics Committee, may not serve as chair/ranking Member of more than one subcommittee. Appropriations subcommittee chairmanships are not exempt.
• The chair/vice chair of the Ethics Committee may serve on no more than two standing subcommittees.
• A Senator may not serve as chair/ranking Member of more than two subcommittees. Democrats interpret this as a "two gavel rule," one full committee chair/ranking Member and one subcommittee, or two subcommittees.
• A Senator shall not serve more than six years as chair of any standing committee, effective January 1997, plus six years as ranking Member of a committee. Once a Senator served six years chairing a committee, the term would be over. However, if a Senator served six years as a ranking minority Member, the Senator could serve as chair if the party controls the chamber.

Limitations on Chair and Party Leader Assignments:

• A chair or party leader may not select an "A" committee on their third round committee selection.