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Fourth Suit Forcing

When an auction starts with three bids in different suits, it is highly unlikely that the fourth suit will be the right strain to play in. If you accept that hypothesis, you can improve the language of constructive bidding considerably by using the fourth suit in such auctions as a cipher. Fourth-suit forcing means that the bid of the fourth suit (normally by responder) simply asks opener to define his hand more accurately. The options for opener are to rebid his suit with extra length, to support partner or to bid notrump with a stopper in the fourth suit. Bidding the fourth suit shows extra values, and in the U.S. it is customary to play it as game-forcing. Some pairs play the auction 1♣ – 1 – 1 – 1♠ as natural, reserving 2♠ as the fourth suit. 

The advantages of playing this method are two- or threefold.  In the first instance it allows you to clearly differentiate between invitational and forcing sequences. So, for example, all of responder’s secondary jumps (in clubs, hearts or spades) after 1♣ – 1; 1♠ are invitational. To create a force, responder uses Fourth Suit then bids one of those suits.

The second big advantage is the ability to find out scientifically whether it is right to play in notrump. After an auction starting 1 – 1♠; 2♣, a jump to 3NT implies no interest in playing elsewhere; a delayed sequence to 2NT or 3NT through the fourth suit is a much more consultative route. Similarly, in the same auction, an immediate 2NT bid shows you have the fourth suit under control. A delayed route implies some concern about the final strain.

By a passed hand, fourth suit is forcing, but not to game.

SOURCE: "Conventions", The Official ACBL Encyclopedia of Bridge, 7th ed. Horn Lake: American Contract Bridge League, Inc., 2011, page 286. Print