The Blackwood convention involves the use of the 4NT bid to ask your partner how many keycards they hold. A keycard is defined as any of the four aces, or the king of trumps. Therefore, there are a total of five keycards.
When should you use Blackwood?
In general, you should use Blackwood to explore the possibility of a suit slam when you are sure that your partnership has enough strength for slam, and either you have agreed on the trump suit already, or you agree now on the suit that partner bid just now.
History and purpose
The original Blackwood convention only asked for aces. The idea was that if you have enough strength for slam but you are afraid that the opponents have two aces, you could use Blackwood to make sure your partnership has at least three of the four aces. Lacking two aces, you then know to sign off in five of the suit rather than six, since opponents can take two tricks off the top by simply leading the suits in which they have the aces.
Most modern players use Roman Keycard Blackwood (the system described here), which counts the king of trumps as the fifth keycard. The idea is that if your partnership has three of the four aces but you're also missing the king of trumps, then you'll lose one side suit trick and must play the trump suit for no losers. With AQ of trumps in one hand, this rests upon a finesse (which has a 50% chance of working). With the A in one hand and Q in the other, the odds are not as good. So it's important to know about the king of trumps in addition to the four aces. If two of the five keycards are missing, then forget about slam.
Roman Keycard Blackwood also allows information to be exchanged about whether the queen of trumps is held. With only four of the five keycards, if you don't have the queen of trumps, slam is risky: you'll probably lose one trick due to the missing keycard, so you need to hope that you can either drop or finesse against the queen of trumps to avoid losing a second one. So often you'll sign off in five of the suit and not take the risk of slam going down due to the missing queen.
When to avoid Blackwood, or at least delay it
- You aren't sure whether or not you have enough strength for slam. With slam-invitational strength, you can instead initiate control bidding.
- The 4NT response would be interpreted by your partner as a quantitative raise of a notrump opening, invitational to slam in notrump (not in a suit).
- There might be a suit in which you and your partner both lack second-round control. In such cases, you must use control bidding to ensure that all suits have second-round control before proceeding with Blackwood.
- There is a risk that the response to Blackwood will both require a sign-off at the 5 level, and be higher than 5 of the suit, leaving the partnership with no escape route.
- This is not a risk when the trump suit is spades.
- If the trump suit is hearts, you had better have at least two keycards yourself. If you have zero or one keycards, you will be very unhappy when you hear the 5♠ response showing only two keycards in partner's hand.
- If the trump suit is a minor, this is even more problematic. Some partnerships don't use Blackwood for minor suit slams at all.
- You have bidding space below 4NT that you can use to either show agreement with partner's suit, describe your hand, or get partner to describe their hand. If you have the space, you might as well use it. For example, here are some alternatives:
- If partner opened 1 of a major, and you have 4-card support, consider responding 2NT or making a splinter raise—both of which show suit agreement—before pulling out 4NT.
- If partner opened 1♥ and you have 3-card support with 4+ spades, respond 1♠ first, which is unlimited and one-round forcing. If partner rebids a minor, then respond with the other minor to force to game (Fourth Suit Forcing) and then raise partner's major to show suit agreement below game level, then bid 4NT. In the meantime, you will learn more about partner's hand.
- If partner opened 1 of a major, and you have 3-card support, another option is to make a 2/1 raise then rebid partner's major to show suit agreement, then bid 4NT. Again, you will be able to establish suit agreement and learn more about partner's hand by doing it this way.
Rebids after 4NT
First, figure out what partner's response means.
- 5♣ shows 1 or 4 keycards.
- 5♦ shows 0 or 3 keycards.
- 5♥ shows 2 keycards but no queen of trumps.
- 5♠ shows 2 keycards plus the queen of trumps.
Our partnership has three or fewer keycards → Sign off in 5 of the trump suit.
- If partner's response is 5 of the trump suit, then just pass.
- If partner's response is above 5 of the trump suit, this is your fault. Remember not to get yourself and your partner into this situation next time. Always practice safe Blackwood.
Our partnership has four keycards and no queen of trumps → Sign off in 5 of the trump suit.
Our partnership has four keycards and I don't know whether we have the queen of trumps → See Queen ask.
Our partnership has four keycards and the queen of trumps → Sign off in 6 of the trump suit.
Our partnership has all five keycards and I am not interested in a grand slam → Sign off in 6 of the trump suit.
Our partnership has all five keycards and no queen of trumps → Sign off in 6 of the trump suit.
Our partnership has all five keycards and I don't know whether we have the queen of trumps but I'm interested in a grand slam → See Queen ask.
Our partnership has all five keycards and the queen of trumps and I'm interested in a grand slam but I need to know about partner's side-suit kings → Bid 5NT.
Our partnership has all five keycards and the queen of trumps, and I see 13 top tricks between our two hands → Sign off in 7NT.
Our partnership has all five keycards and the queen of trumps, and either I know our partnership has all the kings or I know a grand slam will make anyway → Sign off in 7 of the trump suit.