Source: October 2014 ACBL Bridge Bulletin 
Dlr: North; Vul: Both
North opened 3 and East passed. What should South be thinking? South should first look at the vulnerability and then reflect on what kind of preempts partner could make. With North-South vulnerable, South is entitled to consider bidding slam. South saw that if he bid 4NT as Roman Key Card Blackwood (3014), there could be a problem. If North answered 5, showing no key cards, South would stop in 5 and hope that it makes.
If North bid 5 , showing one key card, that means two key cards are missing. If North has the K, your side is missing two aces and slam is likely going down. But you are now in 5 and you can’t get back to 5. If partner shows two key cards, you can bid slam and expect to make it. Are you willing to bid 4NT and risk that partner has less than two key cards?
In this case, the auction might be better if you were using RKCB 1430, where a 5 response would show one key card. But if partner were to bid 5 , showing no key cards, you would be at least one level too high. Blackwood, no matter which version you use, is not the perfect answer.
A modern solution is a key card sequence starting before 4NT. Note that it has two parts. If partner preempts at the two or three level in diamonds, hearts or spades, play that 4 by you asks for key cards. If partner opens 3, which is what he bid on the example hand, use 4 as the key-card ask.
Here is an auction showing the responses.
4 First step – no key cards.
4 Second step – one key card.
4 Third step – one key card and the queen of trumps.
4NT Fourth step – two key cards.
5 Fifth step – two key cards and the queen of trumps.
The first three steps come up the most, but no matter which step partner takes, you should never have a key card accident.
Using the example hand, South would bid 4 over the 3 opening. North has two key cards, the K and the A, so he would bid the fourth step, 5. South can confidently bid slam, concluding a satisfying auction.
This is an easy convention to remember, and it works efficiently. There is, of course, the inevitable question of how it works in competition. What happens if partner opens 3 and RHO doubles? What if RHO overcalls 3 ? I suggest that when RHO bids, you do not play this key-card sequence. If you wish to continue using it, then be sure that your partnership is in tune.