blackwood convention. asking for aces and kings
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 and kings in order 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 named after the Italian team which invented it. I will only cover standard Blackwood here.
The person who instigates Blackwood is generally responsible for placing the final contract. Blackwood uses the bids or 4 NT and 5 NT to ask for aces and kings. These two bids would not normally be heard during normal bidding as one only needs 3 NT to make a game contract. Following the bid of four no trumps the responder replies as shown above bidding 5 clubs if they have no aces, 5 diamonds if they have one and so on up to 5 spades.
The Blackwood instigator can now either place the contract at a high level or sign off if s/he discovers there are not enough aces in the joint holding.
Still using Blackwood they can now bid 5 NT in order to ask for kings with the replies being the same as above but one level higher, i.e. 6 clubs for no kings, 6 diamonds for 1 king and so on.
The problem with Blackwood is that, by the time you have discovered you don't have enough aces and kings you may well be a too high a level. For this reason, many players, (me included), prefer to use Gerber.