A convention in which 4NT is used to discover the number of aces held by partner. It was invented by Easley Blackwood in 1933 and has attained worldwide popularity.
The conventional responses to the 4NT bid are:
5♣ no ace or four aces.
5♦ one ace.
5♥ two aces.
5♠ three aces.
If the 4NT bidder continues by bidding 5NT, he asks for kings in a similar fashion. As this must be an attempt to reach a grand slam, the 5NT bid guarantees that the partnership holds all four aces. At matchpoint duplicate scoring, this idea might be disregarded in the interest of seeking a contract of 6NT.
Some players use the next meaningless bid, instead of 5NT, to ask for kings. This is called Rolling Blackwood.
However, the traditional use of the follow-up 5NT bid to ask for kings has been abandoned by most experts. In Bridge World Standard from 1994, 73% of experts preferred 5NT to be a general grand slam try and a request for a cuebid. Among
American experts, this traditional version of the convention has been largely replaced by Roman Key Card Blackwood (RKCB). When there is no agreed trump suit, RKCB cannot be used. Experts initially favored use of a modified responding scheme:
5♣ = no ace or three aces (0-3), 5♦ = one ace or four aces (1- 4), 5♥ = two aces. Many, if not a majority of players, now use so-called 1430 responses (5♣ = 1 or 4, 5♦ = 0-3).
Requirements
It is seldom wise to use the convention when holding any void or a worthless doubleton in an unbid suit or when matters such as trump quality remain unresolved.
In some circumstances it may be possible to play in 5NT. If the Blackwood bidder next bids an unbid suit at the five level, he is requesting responder to bid 5NT. However, this is rarely necessary because a Blackwood bidder is normally intending to play in a suit.
Void suits
Void suits may not be counted as aces, but there are several methods by which voids can be indicated.
(1) Make the normal response, but at the level of six, to show the indicated number of aces and an unspecified void. Thus 6♣ shows no ace and a void; 6♦ shows one ace and a void, etc.
(2) Bid 6♣ to show one ace and a void; 6♦ to show two aces and a void.
(3) Bid 5NT to show two aces and a void; six of a suit ranking below the agreed trump suit to show a void in that suit and one or three aces; six of the agreed trump suit to show one or three aces and a higher-ranking void.
(4) Holding two aces, make the response that normally shows no aces; holding three aces, make the response that normally shows one ace. When the 4NT bidder signs off, the responder does not pass, but now bids the suit of his void. Responses at the six level show one ace and a void, as in (3).
(5) Using a three-step set of normal responses to Blackwood in which 5♣ shows 0 or 3 aces, 5♦ shows 1 or 4, and 5♥ shows two aces, make a bid higher than 5♥ to show a void. 5♠ shows a spade void and one ace; other responses are as in (3) above.
See also: Key Card Blackwood; Roman Key Card Blackwood; 1430
SOURCE: "Conventions", The Official ACBL Encyclopedia of Bridge, 7th ed. Horn Lake: American Contract Bridge League, Inc., 2011, page 270. Print