In the partnership card game contract bridge, the Blackwood convention is a bidding convention developed by Easley Blackwood in 1933 and still widely used in the modern game. Its purpose is to enable the partnership to explore its possession of aces, kings and in some variants, the queen of trumps to judge whether a slam would be a feasible contract. The essence of the convention is the use of an artificial 4NT bid made under certain conditions to ask partner how many aces he has; responses by partner are made in step-wise fashion to indicate the number held.
Blackwood's original summary
After developing the concept in 1933, Easley Blackwood submitted an article proposing his slam-seeking convention to The Bridge World magazine but it was rejected. Nevertheless, it gained awareness and use amongst players and was written about by several authors. In his own first publication on the convention in 1949, Easley Blackwood comments on the entries in books by others and noted that "...in every one of these books, they have it wrong!" He pointed out several misconceptions and concluded with a fifteen-point summary of the "complete and official" Blackwood Slam Convention. A synopsis of that summary follows: In order to make the 4NT ace-asking bid, you must first: think your partnership has sufficient strength for a slam, expect to be able to make at the five-level even if partner has no aces, and be prepared with a sound rebid no matter how partner responds Partner's responses to the 4NT ace-asking bid are made in step-wise fashion: 5 to indicate 0 or 4 aces 5 to indicate 1 ace 5 to indicate 2 aces 5 to indicate 3 aces When responding, do not count a void as an ace. Generally, 4NT is ace-asking when your side has bid a suit. There are exceptions: when notrump has previously been bid by partner and he subsequently removes one's four-level suit bid to 4NT when a previous opportunity to employ 4NT as ace-asking has not been taken When clubs are expected to be trumps, one must have at least two aces to employ 4NT ace-asking and when diamonds are to be trump, one must have at least one ace. The 4NT bidder is only partial captain of the auction and responder has certain rights: the 4NT bidder sets the level of the contract but partner may correct the denomination when responder has a void in the suit in which he would convey, at the five-level, the number of aces he possesses, he may jump to six of the void suit to convey both the number of aces and the location of the void. when responder has a void and his ace-showing response is in a suit of lower rank than the trump suit, he first tells partner the number of aces he has. If partner signs-off at the five-level, responder may continue to slam if his holding justifies it. If the 4NT bidder, after hearing partner's response, bids a previously unmentioned suit, responder must bid 5NT to end the auction. An ace shown by a cuebid by either partner should not be counted in responding to the 4NT ace-asking bid. A 5NT bid after a response to a 4NT ace-asking bid, asks for kings. Partner's responses to the 5NT king-asking bid are made in step-wise fashion: 6 to indicate 0 kings 6 to indicate 1 king 6 to indicate 2 kings 6 to indicate 3 kings 6NT to indicate 4 kings To ask for kings via 5NT, one must first ask for aces via 4NT even when possessing all four aces oneself. A jump to 5NT without employing the 4NT ace-asking bid is the Culbertson Grand Slam Force and obliges partner to bid the grand slam if he holds two of the three top trumps and a small slam if he does not. If the opponents interfere after the 4NT ace-asking bid, a Pass by responder indicates no aces, the suit one above the opponent's indicates one ace, two above indicates two and so on. Except in duplicate, the king-asking 5NT bid assures partner possession of all four aces. After a 5NT king-asking bid, neither partner is captain and either can set the final contract.
Variations based on 4NT as asking
Several versions of Blackwood are available: Standard Blackwood, Roman Blackwood and Roman Key Card Blackwood (RKC or RKCB). All 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.