A convention invented by Ely and Josephine Culbertson but was first published in the US by Josephine in The Bridge World and hence often referred to as ‘Josephine’, whereby a direct bid of 5NT (not preceded by 4NT) after a trump suit has been agreed asks partner how many of the three top trumps honours (ace, king and queen) he holds.
Originally, responder bid seven of the trump suit with two or three top honours and six of the trump suit with none or one. Later methods give better definition, for example if spades are trumps:
6♣ No top honour.
6♦ One top honour.
6♥ Two top honours.
6♠ Three top honours.
Here is a hand on which you might use the Grand Slam Force:
If partner opens 3♠ (or shows a long spade suit by some other sequence), you want to reach 7♠ unless a top trump is missing. You find out by bidding 5NT.
