When we first start playing, an uncontested bidding sequence in which a partnership bids all four suits, such as 1♣ – 1♥ – 1♠ – 2♦, is  considered completely natural. The opener has shown clubs and spades, and the  responder has shown hearts  and diamonds.   After a little while, we learn  that in this situation it is actually unlikely that there is a  suit fit, so the responder,  instead of showing his diamonds, may just as well bid  either 2NT (invitational to  game with 10-12 points) or  go straight to 3NT (with 13-15 points).  This leaves the bid of the fourth suit (in this case 2♦)  free to be used conventionally as a forcing bid, saying  nothing about the fourth suit,  but simply asking opener to  describe his hand further.  This very useful convention is known as 4th-Suit Forcing  (4SF).  Responder should have opening values. For example with a hand such as:  ♠75 ♥QJ1083 ♦A43 ♣AK9 in the above sequence, any of  3NT, 4♥ or 5♣ might be the  best final contract. To decide which, responder needs to find out more about opener's  hand, so he bids 2♦ as 4SF.   In reply, opener should choose the bid which best  describes his hand. First   priority is to show partner if you have three cards in his  major suit; second priority is  to bid NT with a stop in the 4 th  suit. What should opener bid with the following?

A: ♠KQJ8 ♥K72 ♦9 ♣QJ854  

B: ♠AK98 ♥7 ♦K97 ♣QJ873  

C: ♠AK98 ♥K7 ♦97 ♣QJ873  

With A bid 2♥ to show three hearts and minimum strength. Responder can now happily go on to 4♥ knowing there is an 8-card fit.  

With B bid 2NT to show a stop in diamonds but a minimum point count.   Responder will go on to 3NT.  

With C bid 3♣ to show five or six clubs but no diamond  stop. 5♣ is the safest contract.  

You need to agree with your regular partner whether to  play the convention as forcing to game or forcing for one  round only. 4SF can also be used with stronger hands to find out more about partner's hand with a view to finding  the best denomination for a  slam.  Finally, beware of the exception: the sequence   1♣ – 1♦ – 1♥ – 1♠ is not normally played as 4SF. The   1♠ bid is natural showing spades, but you need to agree with partner whether to  play it as forcing for one  round. The 4SF bid in this sequence is 2♠.