Fourth suit forcing

In the game of bridge, fourth suit forcing (also referred to as fourth suit artificial) denotes a partnership agreement that allows responder to create a forcing auction, at the 2nd turn to bid. Under the "fourth suit forcing convention", a bid by either player in the fourth (unbid) suit is conventional (i.e. does not promise any particular holding in the suit bid). It implies that the bidder has no good bid, but nonetheless has something of value, and wishes to continue searching for a contract. It returns the bidding to their partner, and asks them to find a bid or to further describe his/her hand.

This convention was introduced by the British bridge author Norman Squire, and is adopted by the vast majority of partnerships that play bridge at competitive levels. The "fourth suit forcing" convention is particularly useful on strong (game-going hands) on which no natural forcing bid is available, hence it is a type of game trial bid.

Example

A typical "4th suit forcing" situation is as follows:

North holds::Spades A Q 8 6 2:Hearts 8 4 3:Diams Q 7:Clubs A 6 5

After 2Clubs, North can see there are likely to be sufficient points for game, but he has no good bid::* He has shown his spade suit fully. To rebid spades would imply a longer or stronger suit than he has. They are not good enough for a 3Spades rebid.:* He cannot bid in support of either minor suit because his holding in both is inadequate.:* He cannot bid no trumps because it implies a Heart stop, which he lacks and his partner is quite likely to lack as well (because he's already shown two suits so far). If he bids no trumps they will probably end up in 3 NT and a heart lead will be an obvious strategy: they may well lose 4 or even 5 heart tricks immediately.

North is also unable (or very reluctant) to pass because with three suits bid and around 24-26 + high card points, he feels that there is a good chance some game contract could be viable. But he doesn't have a good bid to make; all the bids available will give his partner the impression of stronger support than he has, and might well lead to a bad choice of contract.

North instead bids the fourth suit ("fourth suit forcing")—2Hearts—to indicate that he believes they have values, and to indicate that he "lacks" the stop in the fourth suit needed for no-trumps, and hasn't got a viable bid in any of the other suits. Depending on partnership agreement, this 2Hearts bid may be either forcing for one round, or forcing to game level. His partner then understands the situation better, and can try to find a bid on that basis instead of being misled.

Further details

Opener responds to the "fourth suit forcing" by (in prioritised order):
# Raising of responder's 1st bid suit with 3-card support,
# Bidding notrump with values in the fourth suit,
# Raising the 4th suit with 4-card in that suit,
# Making the most natural rebid possible, lacking any of the above.

The "fourth suit forcing" approach in conjunction with the principle of fast arrival allows the partnership to create a game-forcing auction at low level that leaves ample room to explore for slam:

Here, the 2Spades bid denotes a four card spade support, and a hand too strong for a fast-arrival bid of 4Spades.