The ACBL official convention card

In the game of contract bridge, a convention is an agreed-upon meaning for a call (a bid, double or redouble, or a pass) during the auction phase of the hand. Often, the inventor of the convention gives it a name; some widespread conventions got a name after their (perceived) authors.

Conventional opening leads and discards may also be used. The term, however, usually denotes just a bidding convention.

Partnerships must agree on conventions beforehand, and must disclose all conventions to their opponents. If they fail to do so, this can be considered an illegal transfer of information.

Conventions may make use of natural or artificial bids. A natural convention, for example, is the one notrump opening (showing a balanced hand and 10-13, 11-14, 12-14, 15-17, 15-18 or 16-18 high card points, depending on the partnership and perhaps other factors). An example of an artificial convention is the Stayman convention, in which the responder's 2♣ response to a 1 NT opening says nothing about the clubs in responder's hand. (In this case, the convention asks opener for further information on his major suit holdings.) The term "treatment" is often used for a "natural convention" and reserve the term "convention" strictly for an artificial bid or response.

The most widely-known and used conventions are Blackwood and Stayman. Other popular conventions are Drury, Jacoby transfers and the strong 2♣ opening. In the strict sense, takeout double is also a convention, but its use is so old and widespread that it's considered an integral part of the game.

Under the rules of the sponsoring organization (national federations such as ACBL, zonal organizations, or World Bridge Federation for international events), certain conventions are alertable, meaning that the partner of the player making a conventional call or play must say "alert" before the right-hand opponent calls or plays. The right-hand opponent may ask the alerter about the meaning of the convention, or can simply proceed as usual. If the right-hand opponent does not ask about the convention, her partner may do so when it is her turn. In ACBL, a few conventions are also announced, such as Jacoby transfers and 1NT openings. When a player uses a Jacoby transfer, his partner simply says, "transfer". When a player opens 1NT, the partner announces the high card point range.

Sponsoring organizations can require players at all or some levels of competition to have a convention card. The convention card is a form which must be properly filled in by the partnership, and contains general notes of the system, and bidding, leading and discarding conventions. In ACBL-sanctioned games, all pairs are obliged to have it, and both members of a partnership must have identical ones. On the ACBL convention card, alertable conventions are shown in red and announced conventions are shown in blue.