There are some deals, where the player with a monster of a hand needs only to find his partner with just one key card to bid a grand slam. Why just the other day I was dealt : (void) AKQx....AKxx....A109xx, and lo and behold my RHO opened the bidding with a pre-emptive 3S. Naturally I doubled, and partner responded 4C. Well, even with 4 small clubs and a favourable 2-2 trump split, 6C looks a good bet. Four to queen offers grand slam prospects, but only if there is a stiff King under the Ace, providing a marked finess of the jack on the way back. However, four to the King either requires a 2-2 split for the grand, or for one opponent to have a stiff honour with falls under the King, enabling the other honour is picked up on a subsequent finesse. Five to the queen could produce all thirteen tricks if the play of the queen finds Kx under the Ace , pinning the stiff jack in the process. But if the King is a singleton, then a low club to the Ace does the business. Best of all is when partner's holding is 5 to the King with the outstanding trumps breaking 2-1, for now the grand is an absolute certainty.
So how best to proceed ? The answer is simple : use Exclusion Key Card Blackwood. A bid of 5S asks responder to show how many of the key cards he has ( Aces plus king of trumps ), but ignoring the Ace in the cue-bidded suit. Once partner flags up one ( that is the King of trumps ) the small slam is near rigid, and the big one could be bid with some measure of optimism and expectation. As it happened my partner denied having the King and so his response of 6C was passed out. When he showed up with a 3-2-3-5 distribution, with the queen and 4 small trumps, I was careful to play the Ace of clubs first. Thankfully, both opponents followed suit, and with the doubleton King off side, there was only the one trick to lose. Not surprisingly in a mediocre field of 24 pairs, there was just three of us who reached slam......... simply because we all used Exclusion Key Card Blackwood.