The Grand Slam Force is a bidding convention in contract bridge that was developed by Ely Culbertson in 1936. It is intended to be used in cases where the combined hands of a partnership are so strong that a slam (winning at least 12 tricks) is a near-certainty and a grand slam (winning all 13 tricks) is a possibility. It allows one partner to gain information on the quality of trumps in the other partner's hand.
When this convention is in force, a bid of 5NT (notrump), when it does not conflict with other conventions used by that partnership (e.g., a 5NT Blackwood bid), is forcing to slam in the suit last bid, unless the trump suit has already been agreed on earlier in the auction. The partner of the 5NT bidder bids as follows:
For example, if a partnership is using the Grand Slam Force and one member bids 1♥ his or her partner bids 5NT, it is implied that hearts are the desired trump suit. The original bidder will sign off in 6♥ with one of the top three heart honors, and bid 7♥ with two of the top three heart honors.
As with all bidding systems, the Grand Slam Force has its limitations, not the least of which is that the chances of employing it directly over an opening one-bid are extremely rare and 5NT must be understood to be the GSF in other situations. Also, numerous variations on this convention have been developed.