The Drury convention in bridge is a convention used to show a game-invitational major suit raise (often called a limit raise) by a passed hand. In the simplest form, a response of 2 clubs, by a passed hand, to a 1 heart or a 1 spade opening, is artificial and shows three or more cards in opener's suit and ten or more support points. (Responder's raise is limited to less than an opening hand, of course, because of the original pass.)

The Drury convention is valuable because:

1. Since you called first and passed, your partner has opened in third or fourth seat, and therefore may have opened "light" (that is, with slightly less than normal opening values). Opening light in the third seat is a valuable tactic, since the player in the fourth seat almost assuredly has the best hand at the table, and may very likely open the bidding if given the opportunity; a third seat opening will therefore effectively preempt the player in fourth seat.
2. Most partnerships play that when a player opens in third or fourth seat, she may pass any response; that is, the passed hand has no forcing bid available. Thus, it is important to describe your hand in a single bid.
3. The methods used to show a limit raise by an unpassed hand normally involve jumping to the 3-level. If partner has opened light, this may be too high even though you have good support.

Opener's Rebid

In the original Drury convention, if opener rebid 2 diamonds, it showed a light opening. Today, most people play Reverse Drury and a rebid of 2 of opener's original major suit shows a light opening (that is, no game interest) and responder is expected to pass. Any other rebid by opener confirms a full opening hand (or better) and shows game interest. With a good hand, say 15 or more points, opener may simply jump to game (4 of her major suit). Other bids tend to be natural and descriptive, in effect a game try. Of course, with an excellent hand, opener may be interested in a slam and will bid accordingly.

Two-way Drury

In this variant, the passed hand with 10+ points responds 2 clubs to show exactly three-card support and 2 diamonds to show four- (or more) card support. This may help opener evaluate the probability of a successful game contract.

Real club (or diamond) suit

If the passed hand has 10+ points and a real club suit (or a diamond suit, when playing two-way Drury), she cannot show it naturally at the 2-level. Some people play that a jump to 3 clubs (or 3 diamonds) shows this hand. Others agree to use the forcing notrump.