Byzantine Blackwood is a bidding convention used in the game of bridge. It is a complex variation of the Blackwood convention devised by Jack Marx of Great Britain which uses a four notrump (4NT) call to enquire about the honor card holdings of partner.[1] Its principal premise is that an ace-showing response to 4NT may safely be based on a holding in which one ace is replaced by a king, so long as the king is in a defined key or half-key suit, and there are not more than two such suits.[2]
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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:
| Bid | Meaning when one key suit | Meaning when two key suits |
|---|---|---|
| 5♣ | 0A; 3A; or 2A+kK | 0A; 3A; or 2A+kK |
| 5♦ | 1A; 4A; or 3A+kK | 1A; 4A; or 3A+kK |
| 5♥ | 2A; A+kK+kQ; or kA+kK+hK | 2A; k(A+K+Q); or A+2kK |
| 5♠ | 2A+kK+kQ; or 3A+hK | 2A+k(K+Q); 2A+2kK; or A+kK+k(K+Q) |
| 5NT | 3A+kK+kQ; or 4A+kK | 3A+k(K+Q); 3A+2kK; 4A+kK; 2A+kK+k(K+Q); or A+2k(K+Q) |
Table legend:
According to this, when there is only one key suit, a Byzantine 5♣ response shows 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, let's say 5♥, its meaning depends upon the prior bidding sequence:
| Opener | Responder |
|---|---|
| 1NT | 2♦* |
| 2♥ | 4NT |
| 5♥ |
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.
| Opener | Responder |
|---|---|
| 1♥ | 1♠ |
| 3♠ | 4NT |
| 5♥ |
Here there is a key suit and one half-key suit. The 5♥ response shows any two aces; ♥AKQ; or any ace plus ♠K and ♥K.
Responses are given in the style of Roman Key Card Blackwood and may be based on a key-suit king instead of one of the aces normally shown. Key suits include:[3]
Byzantine Blackwood is not used when there are more than two key suits.
| Opener | Responder |
|---|---|
| 1♠ | 2♦ |
| 2♥ | 4NT |
| 5♥ |
Here there is a key suit (hearts, agreed by implication) and two half suits (spades and diamonds). So, according to partnership agreements, it is plain Blackwood showing two aces or RKCB showing two keycards without the queen of trumps.
Byzantine Blackwood is not used in the first round of bidding, i.e., 1♥ - 4NT, as responder may only be interested in aces. Partnership agreement is required on whether the 4NT call is Blackwood or RKCB. Many expert pairs employ a direct bid of 4NT as regular Blackwood, as recommended by Marty Bergen.[4]
Cuebidding may follow the response of four notrump. In addition, after a response to four notrump has been made, a bid of five notrump asks for additional high card features. A king or doubleton king-queen not already shown in response to four notrump counts as one feature; a guarded king-queen combination not already shown count as two features. The responses are: