The Blackwood convention is a popular bidding convention in contract bridge that was developed by Easley Blackwood Sr.. It is intended to be used in cases where the combined hands of a partnership are so strong that a slam is a possibility. It allows one partner to gain information on the number of aces, and possibly the number of kings, in the other partner's hand.

When this convention is in force, a bid of 4NT (No Trump) asks the partner to provide information on the number of aces in his or her hand. With no aces or four aces partner replies 5♣; with one ace, 5♦; with two aces, 5♥ and with three aces, 5♠. The asking bidder usually has one or two aces, so it is easy to discover the partnership's combined assets. A continuing bid of 5NT 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.

Beginners?and even more advanced players?often fail to comprehend the fundamental purpose of the Blackwood convention. They believe?incorrectly?that the convention is designed for the purpose of ascertaining if the partnership holds all four aces. In fact, the purpose of Blackwood is fundamentally to determine if the partnership is missing two (or more!) aces. If the partnership is missing only one ace, then 12 tricks are still attainable, assuming that the partnership resources are sufficient to capture this many tricks.

Blackwood should not be used when the information gleaned will not answer the question that needs to be answered. A simplified, but instructive, way to think about Blackwood is this: "I am concerned that we may lose the first two tricks, if we bid a slam. I can use Blackwood as a kind of insurance policy, to guarantee that this will not happen." But Blackwood will not help if, due to the structure of the hands, there are multiple ways to lose the first two tricks. It only helps, for the most part, if the exclusive risk of losing the first two tricks is due to the opponents' holding two cashable aces. Obviously, the opposition might hold the ace and king of a side suit, and could bang those tricks right down, resulting in an immediate set.

Thus, a player should use Blackwood only when he can ascertain that the partnership holds at least second-round controls in all suits (kings or, if a suit fit is found, singletons). Thus, a Blackwood query by the player holding two quick losers in a side suit is a wild gamble, as it is still possible that the suit is not controlled by an Ace or a King.

For the same reason, it is generally wrong to use Blackwood with a void. (This is not always true, but the author's rule is: Don't use Blackwood with a void unless you are absolutely sure you know what you are doing, and why you are doing it. If you don't understand why it is correct, in a given case, to use Blackwood with a void, then it's very likely that its usage will be incorrect.) You may be missing two aces, but your void may compensate for the lack of one of the enemy aces. Thus, Blackwood will not tell you what you want to know: Are we at risk of losing the first two tricks? If your side has two aces and a void, then you are not at risk of losing the first two tricks, so long as (a) your void is useful (i.e. does not duplicate the function of an ace that your side holds) and (b) you are not vulnerable to the loss of the first two tricks in the fourth suit (because, for instance, one of the partnership hands holds a singleton in that suit or the protected king, giving your side second round control).

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 adage is 'don't use the convention if there is a possibility you won't like the reply.'

Roman Blackwood

A variation of the convention, known as Roman Blackwood, was popularized by famous Italian Blue Team. In Roman Blackwood, the responses are even more ambiguous, but more space-conserving. The basic outline of responses is:

5♣	? 0 or 3 aces
5♦	? 1 or 4 aces
5♥	? 2 aces

In practice, the ambiguity is unlikely to occur, as a strength difference between hands with 0 or 1 and 3 or 4 aces is big enough that it can be established in previous rounds of bidding. In other words, a partner who has previously shown e.g. 12-15 range of high points is unlikely to hold 3 aces for his bid, etc.

Even Roman Blackwood convention has several variations, revolving around 5♥ and 5♠ responses. In all variants, they denote 2 aces. One variation is that 5♠ shows extra values, while 5♥ does not. In other variations, responses 5♥-5NT denote specific combinations of aces (same color, same rank, or "mixed").

If the querying partner ascertains that all aces are present, he can continue as follows:

• 5NT is a Grand slam force
• The first available bid which is not the agreed suit is the Roman Blackwood for kings. The partner responds stepwise, as above.

Roman Key Card Blackwood (RKCB)

In modern times, a system called Roman Key Card Blackwood (RKCB) has largely replaced the original sytem, at least among more advanced players. The king of trump is included as a control or a "key card"? in effect, as a "fifth ace"? and so more information is gained. The responses are basically the same as for Roman Blackwood, but with five "aces" in play, and aditionally queen of trumps:

5♣	? 0 or 3 key cards
5♦	? 1 or 4 key cards
5♥	? 2 key cards without trump Queen
5♠	? 2 key cards with trump Queen

As with Roman Blackwood, for the ambiguous answers in the minor suits the asking partner can almost always work out which it is by looking at the controls in his or her own hand and by analyzing the bidding. The response of five key cards does not exists, as it is simply forbidden for the partner lacking any key card to query Blackwood. The old bridge joke that applies here is, if your partner used RKCB (or regular Blackwood), and you have all four aces (or all five key cards, playing RKCB), then don't respond at all. Instead, rise slowly from your seat and leave the room quietly, because you are playing with a lunatic and your life may be in danger.

Even if partner gives a minor suit response to the RKCB 4NT inquiry, the inquiring partner may still determine if his side holds the queen of trumps. Bidding the next "meaningless" suit up from the 5-level response of the interrogated partner is a "queen ask" for the queen of trump. It is interesting to note that, in case when one player can ascertain that a 10-trump fit exists, the queen of trump is considered to be held even if it isn't, because two rounds of trumps will draw all the outstanding trumps 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 4NT bid is considered to be the agreed trump suit for responding purposes. When No Trump was the last bid made, 4NT is considered to be a quantitative raise and invitational to a small slam. Therefore a bid of 4♣ (Gerber) is used in many partnerships to ask for aces in no trump sequences.