Once a partnership is in the slam zone, either player might use Blackwood.

"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. Blackwood should not 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.

What is RKC and why use it?

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 (aces and the trump king).

RKC stands for Roman KeyCard Blackwood. 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? Therein lies the essence of "keycards." 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
Q 5
7 4 2

Answer: 5 to show 1 Keycard (that "key" card is the K).

Q 10 9 3
K Q 4 2
K 5
7 4 2

Answer: 5 to show 0 Keycards

K 10 9 3
A K 4 2
J 5
7 4 2

Answer: 5 to show 2 Keycards (K, A, no trump queen)

A Q 10 9
A 4 3 2
J 5
7 4 2

Answer: 5 to show 2 Keycards (A, A trump Queen)

Notes:

• Some players invert the responses of 5 and 5 (they use 5 to show 0 or 3 and 5 to show 1 or 4). This was the way the convention was first published (it was more in line with the responses to "regular" Blackwood). The method shown at the top of this article (5 = 1 or 4, 5= 0 or 3) is becoming the more popular way. Sometimes, it is referred to as "1430" -- because the responses in order show 1-4 and 3-0. Also, 1430 is the score for 6 or 6 vulnerable. This method (1430) is recommended, because the 5 response (1 ace) comes up much more frequently than the 5 response (0 aces). It is best to have more room for follow-ups (and to stop in 5 if clubs are trump).
• After the 5 and 5 response, opener can ask for the trump queen. The most common way is to bid the next step (5 over 5, 5 over 5) to ask. The responder then signs off in 5-of-the-trump suit if he lacks the trump queen. With the trump queen, he jumps to 6 of the trump suit (if he has no kings to show). With the trump queen and kings to show, he bids the cheapest suit in which he has a king. This queen-ask mechanism confuses most intermediate-level players. I'd recommend avoiding it unless you are quite used to RKC and very experienced.
• 5NT (by the 4NT bidder) asks for kings (aside from the already accounted for trump king). Some people show the number of kings. Others bid the suit in which they have the cheapest king. The former is simpler. The latter is more effective, but complex. Also, the latter is hard to use unless spades (or sometimes hearts) are trump--there isn't enough space.

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.

_{^{Here are 4 practice deals on the subject:}} 

1 Safety First

2 Further Precaution
3 A Bit Greedy
4 Hard Work

Here is Larry introducing the concept at a country club lecture: