Key Card Blackwood (RKCB)
KCB is a version of the standard Blackwood 4NT that’s used to ask partner how many aces he holds. KCB improves on this convention by allowing you to check on five important cards – the four aces and the king of your trump suit.
KCB is “on” after you and partner have agreed on a trump suit. To do that, your auction must have confirmed support with a raise or one of you must have shown great length in a suit. -- before you can assume that 4NT asks about the five key cards. This is one of the most difficult aspects of RKCB to handle, so it's important for a partnership to discuss what types of auctions will confirm a trump suit.
If you bid 4NT directly over a 1-level opening bid, it should be "regular" Blackwood. No suit has been raised, so opener's response shows only the number of aces.
If you have an agreed trump suit, the responses to 4NT give information about five key cards -- the four aces, plus the king of the agreed trump suit. The meanings of responses to 4NT are:
5C -- 0 or 3 key cards
5D -- 1 or 4 key cards
5H -- 2 key cards, but no queen of trumps
5S -- 2 key cards with the queen of trumps
5NT -- 2 or 4 key cards and a useful void
6 of a non-trump suit -- 1 or 3 key cards and a void in that suit (this is only used if the void suit can be shown below 6 of your trump suit)
6 of the agreed trump suit --1 or 3 key cards and a void in a higher-ranking suit
Finding outside kings
After a 5C, 5D, 5H or 5S response, the Blackwooder can continue with 5NT to ask about the number of kings you hold. Since you've already shown (or denied) the trump king, you don't count this when you make your 6-level response to the king-ask. The steps are 6C (no outside kings), 6D (1 king), 6H (2 kings), 6S (3 kings).
If the Blackwooder uses the 5NT king-ask, he guarantees that your side has all five key cards and he shows interest in a grand slam. If you have significant extra strength or an undisclosed source of tricks (a solid side suit, for example), you can accept partner’s grand-slam try immediately (without answering number of kings) by jumping to 7 of your suit.
Finding the trump queen
If responder has 2 key cards, his bid of 5H or 5S will also tell you if he holds the trump queen. You can also get this information after a 5C or 5D response. To do this, the Blackwooder bids the cheapest step (5D over a 5C response, 5H over 5D).
If the Blackwooder uses this step to ask and you do not have the trump queen, you retreat to 5 of your agreed suit. If you do have the queen, you bid the cheapest suit where you have a king.
After you bid your cheapest king, if the Blackwooder has room (and is interested in a grand slam), he can bid a new suit to ask if you have a king in that suit, too.
Here's an example auction:
1S 2NT (forcing spade raise)
4NT (How many key cards?) 5D (I have 1 or 4 key cards for a spade slam.)
5H (Do you have the spade queen?) 6D (I have the spade queen AND the diamond
king, but I don't have the club king.)
6H (I need the heart king.) 7S (I have the heart king, too.)
There are several variations of RKCB responses. The "standard" treatments are the 5C, 5D, 5H and 5S responses, and the Blackwooder's use of the cheapest suit to ask about the trump queen. However, some pairs use different responses to show voids and to answer the queen-ask. Also, some pairs bid specific kings (instead of using the steps) when the Blackwooder uses the 5NT king-ask. You should go over these parts of RKCB with your partner to be sure you're both playing it the same way.