What is "blackwood convention"

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, 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 such a double.