My 18-month old daughter is clearly a genius. At this age, genius means: recognizing which items are which color. If you ask her what color a banana is, she'll tell you (loudly), "Yellow!" Ask about a duckling? "Yellow!" She also knows that the sun, schoolbus, and her favorite socks are all yellow. Brilliant! I'm sure she's right that there are places that have yellow firetrucks as well. 

Whenever we learn something new in bridge, there's a tendency to use it as the answer for everything. Teachers know this. We've seen students open 2 as Conventional response to notrump to ask about majors Stayman or jump to 2 over an opening bid as a transfer to spades. Most of these get resolved after a few mishaps, but the one that persists is 4NT as A system to ask for not only the 4 aces, but the trump king as well Roman Keycard Blackwood.

Don't get me wrong, Keycard Blackood is a great convention. It will prevent you from going to slams missing two keycards (the keycards are the 4 aces and the king of trump). It will also let you know about the queen of trump, another key card (but not Keycard). 

Once we learn Blackwood, it's hard to unlearn it, but just as 2 isn't always Conventional response to notrump to ask about majors Stayman and 2 isn't necessarily a transfer, 4NT isn't always Keycard Blackwood. It can be useful in a variety of situations: takeout, quantitative, or even to play. How do you know? 

Keycard A 4NT bid to ask for aces (or in RKC, keycards) Blackwood requires a trump suit (or else there isn't a trump queen or king). Without one, 4NT MUST mean something else. The most frequent thing it means is "I'm too good to bid 3NT. If you like your hand bid 6NT." That's called a quantitative invitiation. It could also be called a slam invitation (as opposed to a game inviation). Auctions like 1NT - 4NT or 2NT - 4NT are not Blackwood. 

When would you use 4NT as invitational? Partner opens 2NT and you have something like: 

AJ10

K2

KJ109

9872. 

Of course, sometimes, you'd want to use Conventional response to notrump to ask about majors Stayman over the 2NT first. If your hand is 

AJ102

K2

KJ109

987, 

you should respond 3 to 2NT. If opener bids 3 or 3, you can jump to 4NT again. Notice that it's NOT Blackwood, because no suit has been agreed upon. However, if partner bids 3, you need to set trump. The way to do that is by bidding the only suit you can't want to play: 4 (4 and 4 are natural and forcing). If you had hearts and not spades, you wouldn't use Conventional response to notrump to ask about majors Stayman to begin with. After the 4 call, either hand can use A 4NT bid to ask for aces (or in RKC, keycards) Blackwood since spades are now agreed to be trumps. 

 

What about another part of this sequence: 

West	North	East	South
			2
Pass	3	Pass	3
Pass	4	Pass	?

North is showing clubs and a major (we don't know which) and a hand with some slam interest. What should South do with: 

KQ9

KQJ

AQJ42

Q9? 

No fit, just one ace, and lots of diamond values make this feel liek a hand better suited for notrump. 4NT needs to be a place to play rather than cooperative (to cooperate, South can bid a suit).