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, in order to judge more precisely whether slam is likely to be a good contract.
Two versions of Blackwood are common: "standard" Blackwood, developed by Easley, and "Roman key card" or "RKC" Blackwood, named for the Italian team who invented it. Standard Blackwood enables one partner to count partnership aces and kings in general. "Keycard" 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 Keycard 4NT in the one version.
There are other 4NT conventions, such as Norman four notrump, but almost all bridge partnerships deploy some member of the Blackwood family as part of their slam-investigation methods.
If the partnership's preceding call was a natural bid in notrump, then 4NT is also natural. Over an opposing pass it is simply a raise and a "quantitative" invitation to six notrump, a small slam. Over a four of a suit it is likely to be 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 by some analogy to the Blackwood family: 4♣ asks for the number of aces or key cards.
Where both sides are bidding, 4NT is likely to be a conventional takeout asking partner to help choose one of two or three suits, similar to a lower-level takeout double or cue reply to a double.
Where standard Blackwood 4NT is in force, a four notrump bid (4NT) asks partner to give the number of aces in his or her hand. With no aces or four, partner replies 5♣; with one, two, or three aces, 5♦, 5♥, or 5♠. 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.
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