Byzantine Blackwood is a bidding convention used in the game of bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

. It is a complex variation of the Blackwood convention

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...

 devised by Jack Marx

Jack Marx (bridge)

Jack Marx was a British international bridge player who was instrumental in developing the Acol System of bidding.- Life :Marx went to Repton School, and served as a Captain in World War II in the RASC....

 of Great Britain which uses a four notrump (4NT) to enquire about the holdings of partner. 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.

Key suits

Key suits are defined as:

• the trump suit, if one has been agreed upon either specifically or by inference,
• a genuine side suit that has been bid and supported and
• any suit bid by a player whose partner's first bid was in notrump.

Half key suits

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.

Response schemes

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:

A = ace

kA = key suit Ace

kK = key suit king

kQ = key suit queen

k(A+K+Q) = ace, king and queen of the same key suit

hK = half-key suit king

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.

Applications

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:

• the trump suit,
• any genuine side suit bid and supported, or
• any suit bid by a player whose partner's first bid was in notrump.

Not applied

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

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...

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.

Cue bidding

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:

• 6 shows 0 or 3 features
• 6 shows 1 or 4 features
• 6 shows 2 features