The Grand Slam Force is an artificial 5NT asking partner how many top trump honors she holds. It is only used for suit contracts once a trump suit has been agreed or implied.
This convention was originally created by Ely Culbertson and wife Josephine all the way back in 1936. (Ely famously popularized contract bridge in the United States in the 1930s.) From Josephine's book "Culbertson's Contract in Ten Minutes":
"Any free five-no-trump bid in the opening or responding hand is a forcing bid.
Partner must make either of the two following mechanical responses:
- Sign-off at six-odd in the agreed trump suit when not holding AK, AQ, or KQ of that suit. [i.e. 2 of the top 3 honors]
- Bid a Grand Slam in the agreed trump suit (regardless of the strength or weakness of the hand) provided he holds the AK, AQ, or KQ of that suit. [i.e. 2 of the top 3 honors]
"A 'free' five-no-trump bid means a bid which is neither preceded by the same player's four-no-trump bid nor is some conventional response to a previous four-no-trump bid or asking bid."
The Grand Slam Force is useful and easy to remember. A 5NT jump sounds too outlandish for responder to mistake it for anything else, and the responses are very simple. Interestingly, however, the convention did not appear to gain traction until after Ely Culbertson passed away. The great Oswald Jacoby once detailed the convention in his long-standing column "Jacoby on Bridge":1
| ||North|| |
Opening lead - J
"The late Ely Culbertson had one of the most fertile minds in bridge. His writings did much to popularize contract in its early days. One of his ideas that failed to gain acceptance during his life was the "grand slam force."
"This Culbertson bid has gained popularity lately and today is used by almost every expert. It does not come up often, but when it does it certainly makes grand slam bidding easy.
"South has a rockbottom minimum bid, or the nearest thing to one. North doesn't really care much how weak or strong his partner's opening is. All North wants to know is if South holds both the ace and queen of hearts.
"The grand slam force reveals this very simply. He wastes no time in shilly-shallying or getting unnecessary information. All he does is respond five no-trump.
"South is now in the position of the Light Brigade. He is not to reason why, he had just to do or die.
"With the ace and queen of hearts he promptly bids the grand slam. He isn't too happy with the force until he sees the dummy. Then it is all over but drawing trumps and chalking up the score."
- Jacoby, Oswald (October 2, 1963). "Jacoby on Bridge."