What is RKC and why use it?
RKC or Roman KeyCard Blackwood is the modern improved version of Blackwood. The "KeyCards" are the four aces plus the king of trump. Just as we don't want to bid a slam off 2 aces, neither do we want to bid a slam off an ace and the trump king. Such a slam would be at best 50-50--and there is no need to bid it. Also, we would never want to bid a grand slam off an ace. Nor, would we want to bid seven if missing the trump king. RKC is used to make sure we never reach a slam if missing too many "key" cards. In a sense you can treat the trump king as a fifth ace.
There are several variations, but the mainstream method in use today is :
4NT asks for "aces" and then:
5 = 1 or 4 "aces" (note: some players reverse the meanings of 5 and 5)
5 = 0 or 3 "aces"
5 = 2 or 5 "aces" (no trump queen)
5 = 2 of 5 "aces" (with trump queen)
Why the "quote" marks? Remember that instead of "aces," RKC is used to ask about keycards. What are keycards? The 4 aces and the trump king. So, there are 5 "key cards." What is the trump king? There will always be a "trump king." Common practice is to assume the trump suit is the suit the partnership first agreed on (bid and raised). If no suit was agreed, it is assumed to be the last-bid suit. (There are several variations and trouble areas, but usually there is no problem knowing which suit contains the trump king.)
So, after 1-3-4NT, the responder with each hand below would use the chart above to make the response shown:
K 10 9 3
K Q J 2
7 4 2
Answer: 5 to show 1 Keycard (that "key" card is the K).
Q 10 9 3
K Q 4 2
7 4 2
Answer: 5 to show 0 Keycards
K 10 9 3
A K 4 2
7 4 2
Answer: 5 to show 2 Keycards (K, A, no trump queen)
A Q 10 9
A 4 3 2
7 4 2
Answer: 5 to show 2 Keycards (A, A trump Queen)
RKC is a useful tool for experienced players. It is probably the method that causes the most accidents. Be prepared to have some catastrophes if you use this convention.
Once a partnership is in the slam zone, either player might use Blackwood.
Some old-fashioned players might still play "Regular" (or "Plain") Blackwood:
4NT asks for aces and then:
5 = 0 or 4 aces
5 = 1 ace
5 = 2 aces
5 = 3 aces
The asker can then bid 5NT to ask for kings, with the same schedule of replies, one level higher.
This convention has been around since the 1940's. It is a most helpful convention, but is often misused and abused.
Neither RKC nor regular Blackwood should be used as a crutch to determine if there is a slam, but more to make sure that you don't reach a slam off 2 aces (nor a grand slam off 1 ace).
For examples of the typical misuse of Blackwood, please read my article on slam bidding. A good rule of thumb:
If the answer to Blackwood won't tell you if you belong in slam, then don't use it.
Here is Larry introducing the concept at a country club lecture: