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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,〔The Bridge Players' Encyclopaedia, International Edition, Paul Hamlyn, London, 1967, p. 37〕 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 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 reply to a such a double. ==Standard Blackwood== 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 respondent 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Blackwood convention」の詳細全文を読む スポンサード リンク
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