Roman Key Card Blackwood (RKCB) has largely replaced the standard version among tournament players. According to RKCB there are five equivalent key cards rather than four aces; the trump king is counted as one. The key card replies to 4NT are more compressed than standard ones and they also begin to locate the queen of trumps.
5♣ – 0 or 3 key cards
5♦ – 1 or 4 key cards
5♥ – 2 or 5 cards without the trump queen
5♠ – 2 or 5 key cards with the trump queen
Although the replies to 4NT are more compressed, it is almost always possible to infer which number of keycards is correct: 0 or 3, 1 or 4, 2 or 5. Evidence for that inference includes the entire auction as well as the number of key cards that the 4NT bidder holds.
The 5♥ and 5♠ replies with 2 or 5 key cards also deny and show the trump queen, respectively. (Respondent may also show the queen with extra length in trumps, where the ace and king will probably draw all outstanding cards in the suit.)
The 5♣ and 5♦ replies tell nothing about the queen or extra length, but the 4NT bidder may ask about that using the cheapest bid other than five of the trump suit. The code for replies to that "queen ask" vary; a common rule is that the cheapest bid in the trump suit denies the queen or extra length and any other call shows it.
Roman Key Card Blackwood is predicated on existence of a trump suit, which determines which of the four kings and queens respondent should show as key cards.
SOURCE: Wikipedia contributors. "Blackwood convention." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 14 Aug. 2013. Web. 7 Sep. 2013.