Larry Cohen focuses on the aids to slam bidding, starting with a first look at one of the most famous conventions of them all.

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 two Aces (nor a grand slam off one Ace).
A good rule of thumb: if the answer to Blackwood won’t tell you if you belong in slam, then don’t use it.
There are other auctions where some experts use this convention (such as after they’ve overcalled and raised).

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 two 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 7 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 four Aces and the trump King. So, there are five "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:

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 Hearts or 6 Spades vulnerable. This method (1430) is recommended, because the 5♣ response (one Ace) comes up much more frequently than the 5♦ response (zero Ace). It is best to have more room for follow-ups (and to stop in 5♣ if Clubs are trump).
After the 5♣ and 5♦ responses, 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. Once a partnership is in the slam zone, either player might use Blackwood.

Exclusion Blackwood
I can’t believe I am writing this article. I wouldn’t recommend this convention to anyone other than a full- time expert with a full-time regular partner. Still, it is a sexy convention, and I get asked about it all the time. This is a way to ask for Aces outside of a suit in which you are void. For example, suppose you hold: ♠KJ107632 ♥AK ♦– ♣KQJ2.
Your 1♠ opening is raised to 3♠ (limit raise). All you want to know is how many Aces partner has outside of Diamonds. Regular Blackwood (a no-no when holding a void) won’t do you any good. If partner has, say, one Ace, but it is in Diamonds, you have no slam. If he has two Aces, you might belong in 7 (if the Aces are black), but only 6 Spades if one of the Aces is in Diamonds.
By jumping to 5♦ ("Exclusion Blackwood") you are asking for Aces "excluding" the suit you jumped into (Diamonds, in this case). Partner will bid the first step for 0-4, next step for 1, next step for 2, etc. So, 5♥ = 0 Ace outside of Diamonds (note that with only the ♦A, this is the answer partner would give). 5♠ = 1 Ace outside of Diamonds, and so on.

Since anyone reading/using this likely plays RKC, presumably the jump would actually ask for "keycards outside the void suit." So, the answers would be (assuming 1430), Step 1 = 1 or 4 keycards outside the void suit, Step 2 = 0 or 3, etc (you can reverse the order of the replies).

So, when is a bid Exclusion RKC? The asker must jump to a suit above the game level. That unusual bid shows a void and asks for keycards outside that suit. Typically, there will be an agreed trump suit, but if not, it would be in the last-bid suit. You must be absolutely sure the bid can’t logically be natural. Here are some examples:

Warning: Don’t try this at home unless you have a partner who is in on the secret.

This article was written by Larry Cohen and was originally published in Bridgerama+.

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