Svend Novrup has written a 2-part article on RKCB in the December 2006/January 2007 editions of Bridge Plus. He explains the system with examples and gives his recommended version, which I've distilled below.
· Use the 1430 version, i.e. 5♣ shows 1 or 4, 5♦ shows 0 or 3. In his words, one key card rather than no key cards is more frequently found in a hand that has been asked for key cards. If the response with one key card were 5♦ rather than 5♣, it would be impossible to ask for the queen of trumps when hearts are trumps.. . . In other words, the 1430 version is preferred because the 5♣ response showing one key card occurs more often than 5♦ showing no key cards, leaving more room for continued investigation.
· If the 4NT bidder hears 5♣/5♦ and ‘signs off’ in five of the trump suit, responder continues bidding holding four or three key cards respectively, and does so by treating his partner’s bid as a queen ask.
· In response to a queen ask [next suit up over 5♣/5♦ or as above], responder bids as follows:
o Without the queen he bids the trump suit at the lowest possible level
o With the trump queen or compensating length in trumps he bids the suit of his ‘closest’ side-suit king
o With the trump queen but no side-suit king to show, he bids 5NT
o Note that if responder ‘shows’ the trump queen when he doesn’t have it, his partner will be able to draw an inference about his trump length if he himself holds the queen
o This is fairly obvious but, as with cue bidding, by showing his closest king responder is denying possession of any ‘closer’ side-suit kings.
· We already use 5NT to ask for the lowest side-suit king, but Novrup recommends extending this slightly so that any 5-level bid that has no other meaning is asking for the king of the suit bid. For example if the agreed trump suit is spades, partner bids 4NT and you respond 5♣, 5♦ is a queen-ask, 5♠ is a ‘provisional’ sign-off, 5♥ asks for the heart king, and 5NT asks for minor-suit kings. Without the specified king, you return to the trump suit at the lowest available level. Holding the king you bid 5NT with no other side-suit king, else the closest other side-suit king. If hearts is the trump suit, 5♠ is a king ask. If partner asks for minor-suit kings, you bid the lowest you hold, otherwise you return to the trump suit.
· Developing this further, if partner asks with 5NT, you respond 6♣ and he now bids 6♦, you should bid the grand holding the diamond king.
· If partner asks with 4NT, you respond appropriately, and he now makes a direct 6-level bid, he’s asking you to bid the grand if holding that queen. If you don’t have the specified queen but have room to show another queen below the level of the trump suit, you bid that suit. Otherwise you sign off in a small slam.
· If the opponents intervene over 4NT we play regular D0P1, double showing 0 or 3, pass showing 1 or 4, next step showing 2 keycards without the queen, etc. Novrup suggests that if opponents’ intervening bid is above the level of the trump suit, we use DEPO, where double shows an even number of keycards, pass shows an odd number.
· There is a brief discussion of introducing RKCB at lower levels, e.g. if you raise partner’s 1♣ to 3♣, then 4♣ is RKCB. This sequence is somewhat less likely if we’re playing inverted minor raises, but if you raise partner to 2♣ and he rebids 4♣, this is RKCB. In this latter case, I suppose we have the option of making a 3♣ rebid RKCB, preserving the option of retreating to 3NT if things don’t sound too promising on the keycard front?
· Novrup gives an example of Voidwood as a 5♣ rebid by partner after we’ve raised his 1♠ opening to 3♠. Again, the first step response shows 1 or 4 keycards. For us, a direct 5-level rebid after partner has responded 2NT to our 1-major opening is also Voidwood presumably?
Switching to 1430 doesn’t seem much of a leap, especially if it’s the version we play anyway with some partners. I like the additional precision that the 5-level king-asking bids add, and I think my addled grey matter could cope with remembering the responses. With all of these asking bids, we retreat to the trump suit if we don’t have what partner wants [or can’t remember what the devil it means], so the worst that’s likely to happen is that we miss the occasional grand.