Calls made during the auction phase of a contract bridge game convey information about the player's card holdings.
Bill Root defines "convention" as, "A specific agreement between partners to give an unusual meaning to a bid".
Failure to reveal fully the existence and meaning of a convention generally constitutes an illegal communication of information between partners.
Perhaps the most widely known and used conventions are Blackwood, which asks for and gives information about the number of aces and kings held, Stayman convention, used to discover a 4-4 fit in a major suit following an opening no trump bid, Jacoby transfers, used to find a 5-3 fit in a major suit, and strong two clubs to show a very strong hand (usually at least 22 HCP).
Sponsoring organizations may also ban the usage of certain conventions or restrict their use to certain levels of competition; examples are the use of "ambiguous" suits to disrupt opponent's bidding.