Fourth Suit Forcing

Fourth suit forcing (also referred to as fourth suit artificial) is a contract bridge convention that allows responder to create, at his second turn to bid, a forcing auction. A bid by responder in the fourth suit, the only remaining unbid suit, is artificial indicating that responder has no appropriate alternate bid, remains interested in the potential for a game contract and asks opener to bid again to show additional features.

Opener responds to the fourth suit forcing by (in prioritised order):

1. Raising of responder's first bid suit with 3-card support,
2. Bidding notrump with values in the fourth suit,
3. Raising the fourth suit with four cards in that suit,
4. Making the most natural rebid possible, lacking any of the above.

Fourth suit forcing is minimally forcing for one round and usually forcing to game - partnership agreement is required. Whether or not the convention is applicable if the fourth-suit bidder is a passed hand is also a matter of partnership agreement; there is no consensus amongst experts on the options as to its use being non-forcing, forcing, or forcing only after a reverse.

The convention was introduced by the British bridge author Norman Squire and is adopted by the majority of partnerships playing at competitive levels. Useful with strong game-going hands where responder has no natural forcing rebid, it is a type of game trial bid.