Two main styles of Blackwood exist - "standard" Blackwood, and a more sophisticated variant known as "Roman key card" Blackwood, named for the Italian team that developed it. Whilst the former allows inquiry about aces and kings in general, the latter allows precise inquiry about the key cards related to a particular agreed trump suit. In both cases, an initial bid of 4NT (No trumps) is commonly used as the conventional bid to ask partner about their high cards.
The responder should not count a void as an ace. Eddie Kantar recommends bidding the void suit at the 6 level with three aces and a void and 5NT with two aces and a void.
Although alternatives to the Blackwood convention exist (e.g. the Norman four notrump convention), virtually all bridge partnerships deploy a variant of the Blackwood convention (see below) as part of their slam investigation methods.
|5||– 0 or 3 key cards|
|5||– 1 or 4 key cards|
|5||– 2 or 5 key cards without trump Queen|
|5||– 2 or 5 key cards with trump Queen|
Some players also choose to use the club response to show 1 or 4 and the diamond response to show 3 or none; this is referred to as "1430" while the above version is "3014".
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 exist, as it is simply forbidden for the partner lacking any key card to query Blackwood (see below).
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. Usually, if the partner being questioned holds the queen, he will cue a side king, and if not return to the trump suit as cheaply as possible.
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|
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:
|4||– RKCB for clubs|
|4||– RKCB for diamonds|
|4||– RKCB for hearts|
|4NT||– RKCB for spades|
An established partnership will have agreed that as hearts were not supported after opener's rebid, 4 cannot possibly show support, and must be ace asking in diamonds.
It is usually played as the Roman Keycard Blackwood, with only four keycards: 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:
The Blackwood convention is not without problems. When the Blackwood bidder has a void, he 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 contracting for 12 tricks may still be reasonable, 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 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. 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.'
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. The usual situation here is to bid a previously unbid suit instead (if there is one available) - your partner should recognise that this cannot be a genuine bid and corrects to 5NT.