Fourth suit forcing (FSF) means that when your side has bid three suits, responder's bid of the fourth suit is artificial and forcing. It does not promise length in that suit. It says "I have values for at least game interest, but I am not sure where we belong: tell me more."
FSF is standard in almost every natural system: 2/1 Game Force, Standard American, SAYC and Acol all use it. In 2/1 most partnerships play it as forcing to game, which makes the auctions that follow much easier. In SAYC it is forcing for one round.
How Fourth Suit Forcing Works
Take the auction 1♦ - 1♥ - 1♠. Three suits are bid. Responder has an opening hand with five good hearts but no club stopper. 3NT may be right, 4♥ may be right, 5♦ may be right. A natural bid cannot ask the question, but 2♣ can:
| Auction | Fourth suit bid |
|---|---|
| 1♦ - 1♥ - 1♠ | 2♣ |
| 1♣ - 1♥ - 1♠ | 2♦ |
| 1♣ - 1♠ - 2♦ | 2♥ |
| 1♦ - 1♠ - 2♣ | 2♥ |
| 1♥ - 1♠ - 2♣ | 2♦ |
Most often the fourth suit asks for a stopper there, so that notrump is safe. It also gives opener a chance to show three-card support for responder's major or extra length. See what 2♣ means in this auction in every system.
One auction is different: in 1♣ - 1♦ - 1♥ - 1♠, responder's 1♠ is natural, showing four spades. The fourth suit is artificial only at the two level or higher. Many partnerships use a jump to 2♠ in that auction as the artificial force.
When to Use the Fourth Suit
Use FSF when you have game values (or at least a strong invitation in one-round-forcing style) and no natural bid describes your hand:
- You want to play 3NT but have no stopper in the fourth suit.
- You have a five-card major and want to know whether opener holds three-card support.
- You have support for opener's suit and a hand too good for a simple raise, so you force first and support later.
- You are thinking about slam and need a forcing bid that keeps the auction low.
Do not use it when a natural bid says it all. With a stopper in the fourth suit and game values, just bid 3NT. With a fit for opener's first suit and a limited hand, raise.
Opener's Rebids and Responder's Next Step
Opener answers in roughly this order of priority:
- Three-card support for responder's major: raise it. After 1♦ - 1♥ - 1♠ - 2♣, a bid of 2♥ shows three hearts.
- A stopper in the fourth suit: bid notrump. In game-forcing style, 2NT is enough with any strength, because the auction cannot stop below game.
- Extra length: rebid the first suit with six cards, or rebid the second suit with 5-5.
- Four cards in the fourth suit: raise it. Opener may hold four cards there, as in a 4-0-5-4 hand after 1♦ - 1♥ - 1♠ - 2♣.
Responder now has a clearer picture. With a fit, bid game in it or show slam interest. With opener's stopper shown, bid 3NT or raise 2NT. With no stopper anywhere, look for a suit game or a five-level minor contract. Responder's new suit at this point is natural, and a delayed raise of opener's suit after FSF is forcing and shows real support.
Example Deal
South bids the fourth suit, 2♣. North shows a club stopper with 2NT, and South raises to game.
North opens 1♦ and rebids 1♠. South has 13 HCP, five hearts and three small clubs.
South holds 13 HCP: game is certain, the strain is not. A jump to 3NT with ♣763 risks a club run, a rebid of 3♥ shows a six-card suit, and 3♦ would be invitational. South bids 2♣, the fourth suit.
North has no heart support, but the ♣QJ10 make a stopper, so North bids 2NT. South, with a balanced opening hand, raises to 3NT. North declares, East leads, and North's club holding takes care of the suit. A double dummy solver gives eleven tricks in notrump on any lead, more than the ten available in 4♥.
Change North's clubs to ♣ 10 4 2 and the story changes: North would rebid 2♦ to deny a club stopper, and the partnership would stay out of 3NT.
Common Mistakes and Variations
- Passing the fourth suit. It is forcing. Even in one-round style, opener must bid.
- Raising the fourth suit with three small. Opener raises only with four cards there; with three small cards and nothing else to say, rebid a suit or bid notrump with a stopper.
- Using FSF to show the suit. If you really have a long suit in the fourth suit, you usually need a jump or a later rebid to say so.
- Bidding notrump without a stopper. If opener bids 2NT, responder will raise with no stopper of his own. Opener's 2NT promises the stopper.
- Forgetting the one-level exception. As above, 1♣ - 1♦ - 1♥ - 1♠ is natural.
Forcing for One Round
In the older style, still the SAYC default, FSF is forcing for one round only. Responder may have an invitational hand, about 11 HCP, and pass a minimum rebid below game. Opener therefore jumps with extra values: 3NT instead of 2NT with a stopper and a maximum, for example.
In game-forcing style, responder promises about 12+ HCP and the partnership is committed to game. Opener never needs to jump, and responder handles invitational hands with natural bids (a 2NT rebid, a raise to three of opener's suit, a jump in his own suit). The game-forcing version is easier and is the more common choice among club players who play 2/1 today.
After a Two-Level Response
In 2/1 Game Force, an auction like 1♠ - 2♣ - 2♦ is already forcing to game. A bid of 2♥ by responder is the fourth suit, but no extra force is needed. Partnerships treat it in one of two ways: as a natural bid showing four hearts, or as an artificial "tell me more, especially about hearts" bid. Either works; pick one.
In Standard American, where a two-over-one response is forcing for one round only, the fourth suit after a two-level response is a clear game force. Responder has shown 10+ HCP already and now adds the values for game.
The Fourth Suit Forcing Bid Tree
After 1♦ - 1♥ - 1♠ - 2♣ (game-forcing style):
| Opener bids | Meaning |
|---|---|
| 2♦ | Five or more diamonds, no club stopper, no three hearts |
| 2♥ | Three-card heart support |
| 2♠ | Five spades, so six or more diamonds (rare) |
| 2NT | Club stopper, any strength |
| 3♣ | Four clubs |
| 3♦ | Six or more diamonds, extra values |
| 3NT | Club stopper with a maximum, only in one-round-forcing style |
Opener does not need a full stopper to bid 2NT in some partnerships: Qx or Jxx is often enough when responder may hold a partial stopper too. Agree how thin a stopper you will show, and write it down.
Practice: Fourth Suit Forcing
Partner opens 1♣ and rebids 1♠. Your call?
You hold 14 HCP with five hearts and three small diamonds. Partner opened 1♣ and rebid 1♠. What do you bid?
Your hand:
FSF hands are among the most common game-strain decisions you will face. In the BridgeDummy Bidding Room you can bid random deals against EPBot robots, alone or with a partner at /bidding/partner, and check after each deal whether your side found the stopper or the fit. For the theory behind responder's bids, read two over one game force, the related checkback tools new minor forcing and XYZ, and the natural options in responder's rebid.