With all the bells and whistles! Even experienced players may find something new to try.
There is a lot to take on in this post but you don't have to do it all at once. You can start with basic key card asking and add the various bits described below as you progress. They are presented in a sensible order to take them on little by little. If nothing else, partnerships should upgrade from Blackwood to Key Card Blackwood, described first below, since this easily deals with the important king of trumps.
Key Card asking
A key card is any ace plus the king of the agreed trump suit so there are 5 key cards in all. Key Card Blackwood uses the bid of 4NT to ask partner how many key cards they hold. The standard responses are:
5♣ - 0 or 4 key cards,
5♦ - 1 or 5 key cards,
5♥ - 2 key cards
5♠ - 3 key cards
Roman Key Card Blackwood includes information about the queen of trumps by modifying the responses as follows:
5♣ - 0 or 3 key cards,
5♦ - 1 or 4 key cards,
5♥ - 2 key cards without the queen of trumps
5♠ - 2 key cards with the queen of trumps
With regard to the 5♠ bid showing 2 key cards and the queen of trumps, you should show the queen of trumps even when you don't have it but you do know that you have at least a 10 card trump fit since then there is a good chance that the queen will drop or will be finessable.
Strictly, the major suit responses show 2 or 5 key cards but it is highly unlikely that partner will ask with no key cards themselves so you will practically never need to show 5 key cards.
The above responses are referred to as 'RKCB 3014' or 'RKCB 3041'. Those partnerships which use queen asking (see below) usually swap the minor suit responses, however, to leave more space for queen asking when partner shows 1 key card.
5♣ - 1 or 4 key cards,
5♦ - 0 or 3 key cards,
This method is referred to as 'RKCB 1430' or 'RKCB 4130'.
King asking
After key card asking and the subsequent response then 5NT is a grand slam try since it commits the partnership to a small slam anyway. It guarantees that the partnership has all 5 controls, informs partner that you are interested in a grand slam and enquires about partner's king holding outside the trump suit. Knowing that the partnership has all 5 controls may allow the responder to bid the grand slam directly, without responding to the king ask, on suitably distributional hands.
One way of responding, as with normal Blackwood, is to show the number of kings held but this is now a maximum of 3. The king of trumps in not included in your response since it was included in the key cards response.
6♣ - 0 kings
6♦ - 1 king
6♥ - 2 kings
6♠ - 3 kings
A popular alterative method is to respond, not by showing the number of kings held, but by cue bidding the suit of the lowest king held below the trump suit . This can be useful when the grand slam depends upon responder holding a specific king. With no king, or none below the trump suit, responder bids 6 of the agreed trump suit.
Another method, growing in popularity, is for responder holding one king to bid it if it is below the trump suit but, if holding two kings, to bid the king that they do not hold! With no king, or one above the trump suit, they bid 6 of the agreed trump suit.
The 5NT bidder, holding at least one of the outside kings, can easily interpret the response. If the king that the 5NT bidder holds is the king shown by responder then responder is showing the other two kings. If not, then responder is showing the king that they hold. For example: suppose spades are trumps and you hold the king of diamonds. You bid 5NT and partner responds 6♦. This is the king that you hold so partner is showing the other two kings. If he responds 6♣, however, then he holds the king of clubs. Note that the opponents will not be able to work out partner's holding.
Queen asking
After a minor suit response, which says nothing about the queen of trumps remember, you may be able to ask for the queen by bidding the lowest available 5 level non-trump suit. For example: playing RKCB 3041, if spades are trumps and the responder shows 1 key card the auction goes 4NT - 5♦. Now 5♥ asks if partner has the queen of trumps (spades). Without the queen they bid the agreed trump suit at the lowest available level. With the queen and no outside kings they bid 5NT and with one or more kings they bid 6 of the lowest suit in which they hold a king. Playing in hearts, however, 5♥ is a sign-off in the trump suit (though see below). You have to bid 5♠ to ask for the queen of trumps. This commits you to playing at the 6 level and hence is only useful for investigating a grand slam.
You can see now why most high level players use RKCB 1430. With one ace, the key card auction now starts 4NT - 5♣ and you have space to ask for the queen of trumps with 5♦ whether playing in hearts or spades. It is unlikely that you will be interested in a slam when partner shows no key cards so it is better if this is the 5♦ response. RKCB 1430 works best when the 4NT bidder is the stronger hand since it is then unlikely that the responder has 3 key cards and replies 5♦.
Two-way Queen asking
Consider the sequence 4NT - 5♦ - 5♥ when playing in hearts. There is no room for a queen ask as described above. You can play this sequence as a two-way queen ask, however. If responder has the lower of their possible number of key card (0 or 1 depending upon which RKCB method you are using) then it is a sign off but if they hold the higher number (3 or 4) then it is a queen ask and they should reply accordingly. Similarly, 4NT - 5♣ - 5♦ can be played as a two-way queen ask when playing in diamonds.
When opponents double or overcall
If you play normal Blackwood then you might already use 'ROPI' when opponents double your 4NT and 'DOPI' when they overcall it. These conventions can also be used when playing RKCB.
ROPI applies when your 4NT key card ask is doubled. (This is usually a conventional bid of some sort.) You now Redouble with 0 or 3 and Pass with 1 or 4 then continue with 5♣ for 2 without the queen and 5♦ for 2 with the queen.
DOPI applies when opponents bid over your 4NT. You now Double with 0 or 3 and Pass with 1 or 4 then continue with the lowest available bid for 2 without the queen and the next highest bid for 2 with the queen. When opponents overcall in a major, however, this can take you to the 6 level. You may prefer to 'DEPO' - that is double with an even number of key cards (0 is even), pass with an odd number and forgo the information about the queen of trumps.
Note that some people playing RKCB 1430 use RIPO and DIPO (swapping the 0 and 1 responses) for consistency but I am too old to change! I play RKCB 1430 with ROPI and DOPI though I am thinking of changing to DEPO after an overcall of a major having researched it for this article.
Void showing
Ideally, voids should be identified by cue bidding sequences earlier in the auction but the BBO GIB robots playing RKCB 0314 (as they call it in their system notes) include responses to 4NT which go above 5♠ to show hands with voids. 5NT shows an even number of key cards with an unspecified void and 6x shows an odd number of key cards with a void. If 6x is below 6 of the agreed suit then the void is in the suit bid. If 6x is a bid in the agreed suit then the void is in an unspecified higher-ranking suit.
This seems to be quite a playable system but I think it needs to be used with a little care. First of all, I would not treat 0 as an even number of key cards! I would like 2 key cards to show a void in this manner unless the auction has shown that you have very few high card points and partner was still interested in a slam. Secondly, I would suggest that you probably shouldn't show a void in partner's first bid suit since this is unlikely to be particularly useful.
You want more?
I was surprised to find how much more there is. There's the specific king ask, specific suit ask, continuations after void-showing responses and more. Eddie Kantar has written extensively on this topic over the years. His books 'Roman Keycard Blackwood (The Untold Story)', 'Roman Keycard Blackwood: Slam Bidding in the 21st Century' and 'Roman Keycard Blackwood: The Final Word' were published from about 1990 to 2008 and are now out of print though second hand copies are available. They are more than 200 pages in length!
Probably the most extensive free summary can be found on Dan Neil's website on his resources page where 'Kantar's Roman Keycard System' can be downloaded as a .zip file. This is a collection of separate Word files which were produced from a web copy of the system. It's an inelegant presentation but an extremely complete summary with lots of useful examples.