Playing Roman Key-Card Blackwood 4NT asks how many Key-Cards (as opposed to Aces) do you have?

There are 5 Key Cards: the four Aces and the King of Trumps.
If Trumps have not been agreed they are taken to be the last suit bid.
e.g. Spades in both 1♠ -3♠ - 4NT and 1♥  - 2♦ - 2♠ - 4NT auctions.

After 4NT: 5♣ = 0 or 3; 5♦ = 1 or 4; 5♥ = 2 plus the Queen of Trumps; 5♠ = 2 without the Trump Queen.

On this hand that I played online South has 4-card trump support and a Losing Trick Count of 7 which is enough to bid 4♠.
North is too good to pass with 17 High Card Points, and a 5-5-2-1 shape so bids 4NT.

Playing ordinary Blackwood North will find out you ahve all the Aces but still doesn'know about the ♠K.
Playing R(oman) K(ey) C(ard) B(lackwood) South will anwer 5♥ showing both the missing Ace and the ♠ K, allowing North to bid 6♠ with confidence.

The lead is the ♥ J, plan the play.

East's 2NT is 'Jacoby' showing at least game values and 4-card Spade support. This hand is a perfect example of its value in keeping the auction low and giving room to explore a slam.

Different pairs continue after 2NT (Jacoby) in different ways but I like the simple approach that says 'we have agreed a trump suit and are in a game forcing auction so all non-trump suit bids are now cue bids'.

3♣  shows the ♣ A, 3♦ the ♦ A, 3♥ the ♥ A, 4♦ the ♦ K.

Now West can use Roman Key Card Blackwood (explained in the first of this week's hands).

East's 5♠ says I have both missing key-cards (the ♦ A and the ♠ K and I also have the ♠ Q).

Cautious Wests might settle for 6♠ but those bold enough to bid 7♠ make it on the ♥  finesse.