Blackwood is the most well-recognized convention for slam bidding. When a player is interested in bidding a slam but is afraid of missing two aces, and hence two tricks, he can bid 4NT, Blackwood, to ask his partner how many aces he holds. The responses to 4NT are:

5	Shows 0 or 4 aces
5	Shows 1 ace
5	Shows 2 aces
5	Shows 3 aces

All of the rebids made by the Blackwood bidder are natural, with the exception of 5NT. If the Blackwood bidder is contemplating a grand slam and needs to know how many kings his partner has, he can rebid 5NT. The responses to 5NT are:

6	Shows 0 or 4 kings
6	Shows 1 king
6	Shows 2 kings
6	Shows 3 kings

There are a number of ways to show aces if 4NT is overcalled. One way is called DOPI, which stands for "Double = Zero aces, Pass = 1." The cheapest available bid shows two aces, the second-cheapest bid shows three aces, and the third-cheapest bid shows four. Another method of showing aces is DEPO, which stands for "Double = Even number of aces, Pass = Odd number of aces."

Note that on occasion the responder to a Blackwood inquiry will have a void somewhere in his hand. Voids should not be treated as aces; how they are treated instead is up to partnership agreement.