Byzantine Blackwood convention

Byzantine Blackwood is a bidding convention in the game of bridge. Devised by Jack Marx, it is a complex version of the Blackwood convention (by which a four notrump (4NT) call asks about partner's honor card holdings). Its premise is that both aces and kings may safely be shown in response to a Blackwood-type 4NT enquiry; provided that such kings are in well-defined key or half-key suits, there being no more than two such suits. The name was chosen to express the idea that Byzantine Blackwood is a development from Roman Blackwood, by analogy with the Byzantine Empire and the Roman Empire; rather than that the convention is of Byzantine complexity. It seems to be little used nowadays (2014), most experts employing some form of Roman Key Card Blackwood.

Key suits are defined as:

A half-key suit is defined as a genuine suit that has been bid but not supported. When each partner has bid a half-key suit, the suit bid by the four notrump bidder is the half-key suit in which a king may be shown.

Byzantine is initiated by a call of four notrump and there are two scales of responses: one for use when there is only one key suit, and one when there are two. If there is only one key suit, the king of a half-key suit may be shown; if there are two key suits, half-key suits are not shown. The responses to 4NT are:

Table legend:

Thus, when there is only one key suit, a Byzantine 5♣ response shows either: no aces, three aces, or two aces plus the key-suit king. The response when there are two key suits is the same; but when the answer is, say, 5♥, the meaning depends upon the preceding bidding sequence:

If 2♦ is a Game Forcing Stayman or a 2♥ transfer, then 4NT agrees to hearts as trump and 5♥ shows two aces, or one ace and ♥KQ. As there is no half-key suit the third option is invalid.

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