In the partnership card game contract bridge, the Blackwood convention is a popular bidding convention that was developed by Easley Blackwood. It is used to explore the partnership's possession of aces, kings and in some variants, the queen of trumps, to judge more precisely whether slam is likely to be a good contract.
Two versions of Blackwood are common: "standard" Blackwood, developed by Easley in 1933, and "Roman Key Card" Blackwood (RKC or RKCB), named for the Italian team which invented it. Standard Blackwood enables one partner to count partnership aces and kings in general. Key card variants are defined by a particular "key" suit and enable counting the trump king and queen, as well as aces and kings. Both versions are initiated by a bid of four notrump (4NT), and the entire family of conventions may be called Blackwood 4NT in both versions, or Key Card 4NT in the key card variation.
There are other 4NT conventions, such as Culbertson 4-5 Notrump, Norman Four Notrump and San Francisco, but almost all bridge partnerships employ some member of the Blackwood family (which includes Byzantine Blackwood) as part of their slam-investigation methods.
If the partnership's preceding call is a natural bid in notrump, then 4NT is usually played as natural. Over an opposing pass it is simply a raise and a quantitative invitation to six notrump, a small slam. Over an intervening four of a suit by opponents it is usually played as a competitive raise, expecting to play four notrump. Those natural interpretations may hold in other auctions where the partnership has previously bid notrump naturally or shown a balanced hand conventionally. In some situations where 4NT is a quantitative invitation, especially where 4♣ is a jump, many partnerships use the Gerber convention instead of the Blackwood family: 4♣ asks for the number of aces or key cards.
Where both sides are bidding, 4NT is often played as a conventional takeout asking partner to help choose one of two or three suits, similar to a lower-level takeout double or cuebid reply to a such a double.
Where standard Blackwood 4NT is in force, a four notrump bid (4NT) asks partner to disclose the number of aces in his hand. With no aces or four, partner replies 5♣; with one, two, or three aces, 5♦, 5♥, or 5♠, respectively. The difference between no aces and four is clear to the Blackwood bidder (unless the partnership lacks all four) so one member of the partnership knows the combined number of aces. That is often sufficient to set the final contract. (A common agreement is that when spades is not the trump suit, 5♠ asks responder to bid 5NT. That is useful when the reply to 4NT bypasses the intended trump suit but also shows that slam is likely to be a poor contract because two aces are missing.)
The continuation bid of 5NT asks for the number of kings according to the same code of replies at the six-level: 6♣ shows no kings or four, etc. Asking for the number of kings confirms that the partnership holds all four aces, so partner may reply at the seven level with expectation of taking thirteen tricks.
A void may be as good as an ace in some situations but it should not be counted as an ace. Some experts (Kantar for one) recommend the 5NT reply to 4NT - the cheapest with no standard assigned meaning - to show a void plus two aces and six of a suit to show a void in the bid suit plus one ace.
Roman Key Card Blackwood (RKCB) has largely replaced the standard version among tournament players. It developed from the Roman Blackwood variant (see below). 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. (Responder 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. An option is for the positive calls to show a feature, such as a king in that suit, and 6 of the trump suit can show the queen of trumps with no outside kings.
Roman Key Card Blackwood is predicated on existence of a trump suit, which determines which of the four kings and queens responder should show as key cards. Trump agreement is not necessary, however. One common rule is that the last suit bid before 4NT bid is the key suit, lacking trump agreement.
Some partnerships use the club response to show 1 or 4 and the diamond response to show 3 or none, dubbed "1430" (coincidentally the score for making a vulnerable small slam in a major suit), with the original version being dubbed "3014" when distinction is necessary. In order to facilitate the Queen Ask, an experts' version has been developed, where "1430" is used by the strong hand and "3014" is used by the weak hand. There are specific rules which determine when the asker hand is the weak one and when it is the strong one.
A half-way house between standard Blackwood and RKCB is Keycard Blackwood. Again there are five key cards, including the trump king, but unlike RKCB, the queen of trumps is not considered.
5♣ - 0 or 4 key cards
5♦ - 1 or 5 key cards
5♥ - 2 key cards
5♠ - 3 key cards
This is advocated by Bernard Magee as being simpler for club players, as with RKCB players are sometimes unsure whether partner holds 0 or 3 key cards, or 1 or 4.
A variation of the standard Blackwood convention, known as Roman Blackwood, was popularized by the famous Italian Blue Team in the 1960s. 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, for example, 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:
"Kickback" is the variant of RKCB devised by Jeff Rubens in accordance with the Useful space principle. The step responses are the same as in RKCB, but the ask is not necessarily 4NT. Instead it is the 4-level bid immediately above the agreed trump suit; i.e.:
|4♦||- RKCB for clubs|
|4♥||- RKCB for diamonds|
|4♠||- RKCB for hearts|
|4NT||- RKCB for spades|
Kickback has the advantage that it saves bidding space and, especially for minor-suit fits, provides safety at the 5-level if the required key cards are missing. Because the Kickback bid would otherwise be a control bid, 4NT is usually substituted as the control bid in that suit (e.g., 4NT is a control bid in hearts if the agreed trump suit is diamonds). The drawback is that in unpracticed partnerships there can be confusion as to whether a bid is Kickback, a control bid or preference for a different strain:
|♠||K 10 6 4 2|
|♥||K 10 8 6 4 2||♥||A|
|♦||K J 9 7||♦||A Q 10 4|
|♣||8||♣||A 5 3|
East intended 4♥ as Kickback, but West thought it was secondary support for hearts, and decided to pass with minimum values. As result, a reasonable grand slam in diamonds was missed.
An established partnership might have agreed that as hearts were not supported after opener's rebid, 4♥ cannot possibly show support, and must be ace asking in diamonds.
"Redwood" is a variation of Kickback, in which the kickback principle is used only when a minor suit is trumps. The name comes from the fact that the resulting ace/key card ask will be a red suit:
4♦- RKCB for clubs
4♥- RKCB for diamonds
Using "Redwood," the ace/key card ask of 4NT is still used when the trump suit is a major (hearts or spades).
"Minorwood" is a variation of Blackwood, in which the minor suit which the partners agree will be trumps is itself used as the ace/key card ask. The ask will be at the four level. Hence:
4♣- RKCB for clubs
4♦- RKCB for diamonds
Exclusion Blackwood or Voidwood. was devised by Bobby Goldman as an attempt to resolve the situation when the Blackwood-asker has a void. In that case, he is not interested in the partner's ace in the void suit, as he already has the first-round control; partner's ace would present a duplicated value in that case. It should be noted, however, that many players, even experts, refuse to play Exclusion Blackwood because of the potential disaster of forgetting the agreement.
It is usually played as the Roman Key Card Blackwood, with only four key cards: the three Aces outside the void suit and the King of trumps. However, the asking bid is not 4NT, but the void suit - Voidwood is made by jumping on level 4 or 5 in the void suit after a fit has been found, for example:
Bids of 5♣, 5♦ and 5♥ present a Voidwood, denoting the void in the suit bid and asking for other key cards. The responses are, as in RKCB:
When the Blackwood bidder has a void, and partner shows one or more but not all missing aces, "asker" will not know whether partner's ace duplicates the void suit (where it would not be of great help) or covers side suit losers. For this reason cue bid to show aces explicitly, or Exclusion or Voidwood convention to ask for useful aces, is used with hands that contain a void.
Often non-experts ask for aces when they really need other information from partner, acting act as if the convention is designed to ascertain whether the partnership holds all of the four aces or five key-cards. In fact the fundamental purpose should usually be to distinguish between lacking one and lacking two, because lacking two aces makes slam a bad bet. In other words, ace and key-card asking conventions should normally be used by one member of a partnership who plans at least to contract for 12 tricks if no more than one ace or key card is lacking.
A simplified 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 rule of thumb 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. 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 a minor is the agreed trump suit, or the key suit when no trump suit is agreed. 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. In these cases experts agree on using the Kickback or Redwood convention to save steps and be able to stop at 5♣ or 5♦. An alternative is to avoid using any type of Ace asking and use the cue bid alternative.
A further problem occurs when, after hearing his partner's response, the player who bid 4NT wants to stop in 5NT - as this is a forcing bid asking for Kings. A common agreement is that the first non-suit asks responder to bid 5NT. Whenever the first non-suit is a "queen ask", then the next non-suit asks to bid 5NT. This has to be planned in advance as if there is no such space partners have to bid a usually unmakeable slam contract.