The Blackwood convention in contract bridge was developed by Easley Blackwood, a well known North American player in former days, is a means to ascertain the number of aces in one's partner's hand. The bid of 4 No Trump is the asking bid. With no aces or four aces partner replies 5 Clubs; with one ace, 5 Diamonds; with two aces, 5 Hearts and with three aces, 5 Spades. The asking bidder usually has one or two aces, so it is easy to discover the partnership's combined assets. A continuing bid of 5 No Trump asks for Kings with the replies following the same pattern.
This system is not without problems, however. With hands that have a void, a player is not able to tell whether partner's ace is in the void suit (where it would not be of great help) or in a side suit (where it would be very useful.) For this reason cue bidding to show aces is a superior method with hands that contain a void. In fact, most beginner-level players misuse this convention; they ask for aces when they really need other information from partner.
Other problems can easily occur when Clubs is the agreed upon trump suit. The reply to Blackwood could take the partnership past their agreed suit and going to the next higher level may be one trick too high. The antidote is 'don't use the convention if there is a possibility you won't like the reply.'
In modern times, a system called Key Card Blackwood has largely replaced the original sytem. The king of trump is included as a control and so more information is gained. Replies are grouped as well to increase data exchange. In the system known as 14/30, the response of 5 clubs shows one or four controls; 5 diamonds shows three or zero controls. Partner can always work out which it is by looking at the controls in his or her own hand and by analyzing the bidding. 5 hearts shows two controls without the queen of trump and 5 Spades shows two controls with the queen of trump. All this information is very useful. Bidding the next suit up is a queen ask for the queen of trump if it hasn't been shown already. It is interesting to note that where 5-5 trump fits exist, the queen of trump is considered to be held even if it isn't because two rounds of trump will call all the outstanding trump in a very high percentage of the cases. All the foregoing bidding is predicated on the assumption that a trump suit has been agreed upon. Without trump agreement, the last suit bid before the 4 No Trump bid is considered to be the agreed trump suit for responding purposes. When No Trump was the last bid made, 4 No Trump is considered to be a quantitative raise and invitational to a small or grand slam. Therefore a bid of 4 Clubs (Gerber) is used to ask for aces in no trump sequences.
