Suppose spades are trumps and you bid 4NT according to Roman Key Card Blackwood. Partner responds 5 diamonds, saying he has one or four key cards (the key cards are the four aces plus the king of spades). From your own cards, you know it has to be one. You proceed with 5 hearts, conventionally asking if he has the queen of spades.
Your plan is to bid a grand slam or a small slam depending on the response to your question. The odd thing is that you plan to bid a grand slam if partnern denies the trump queen, but only bid a small slam if he has the queen. Why?
There are two technical solutions and one tactical. I have accepted all three of them.
The tactical solution is based on the state of a match, in which a flat board is no better than a big loss. Your goal is therefore to do the opposite from what you think they will do at the other table. That is to reach a grand slam when you miss the queen, but stop in a small slam when you have it.
The first technical solution is that you hope your partner hasn't the queen, since it isn't needed. If you, who ask, know that your side has ten or more trumps, it's better if your partner has a queen in another suit than in trumps, where it isn't needed. Here is a good example from Göran Ofsén. It goes like this:
Suppose North has shown 6-7 HCP and six spades when South has a hand like:
K J x x
A K x
A K x
A K x
South asks for aces and learns that North has the spade ace. If he proceeds to ask for the trump queen, it is better if North denies it (so that it is possible that he has a side-suit queen instead) than shows it (when he can't have a side-suit queen).
The second technical solution is very beautiful. I read about it in the American bridge magazine The Bridge World many years ago. Then, North opened with 1NT (15-17 HCP), when South had roughly this:
A J x x
A K J x
A Q J x
South used Stayman and learned that North had four spades and less than four hearts. He set spades as trumps with a jump to 4 clubs (Splinter), when North (as expected) tried to sign off in 4 spades. South proceeded with 4NT (Blackwood) and learned that North had one key-card. So, the spade king or the club ace was missing.
It looked like South had to be content with a small slam, but if he is clever and asks for the trump queen something good may happen...
But not if North says he has the queen. Then, South doesn't know if the spade king or the club ace is missing, so he has to sign off in 6NT. But if North denies the spade queen, South knows the whole picture. Since North's minimum strength is 15 HCP, he must have the club ace (East-West has at most 5 HCP). So, the defenders' honors are the spade king and the spade queen. That means South can bid 7NT, knowing that his side has one trick in spades plus four trick in each of the other suits. The only way for North to hold 15 HCP is if he has the heart queen, the diamond king and the four top honors in clubs:
10 x x x
Q x x
A K Q J
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