Source: ACBL
Whole books have been written on the topicof ace-asking conventions, so the discussion of  Easley Blackwood’s concept in this column will be brief by comparison. The Blackwood convention, developed early in the history of duplicate bridge, quickly gained popular approval. As many bridge teachers will point out, however, Blackwood may be responsiblefor more bidding disasters thanany other convention developed.
Understanding its proper use, therefore, is important for successful duplicate play. The basic idea behind the Blackwood convention is to determine how many aces partner holds. In certain circumstances, this permits slams to be accurately bid — or avoided. The player who wishes to know how many aces partner holds bids 4NT, usually as part of a constructive auction.
For example:
Opener Responder 1Spade Suit           3Spade Suit 4NT Responder’s 3Spade Suit is a limit (invitational to game) raise. Opener’s 4NT is the Blackwood convention, asking responder to indicate how many aces she holds, using the following schedule of responses: 5Club Suit zero or four aces 5Diamond Suit one ace 5Heart Suit two aces 5Spade Suit three aces If responder held this hand: Spade SuitA 9 7 6 A 10 3 9 4 K 8 6 3, she would bid 5, therefore, to show two aces. This sounds simple enough, but there’s a catch. It’s critical that the 4NT bidder have the right type of hand to ask for aces. First, the Blackwood bidder must have some indication that the partnership’s combined values are in the slam range. Second, the 4NT bidder must have a hand that will allow her to proceed intelligently after hearing the response. This means that the asker should not have a hand with two fast losers in one suit, nor should she have a void.
 Say you held:Spade SuitA K Q 10 5 36A 9 2K Q 7, and opened 1Spade Suit. If partner responds 3Spade Suit(limit), you have an excellent hand for slam purposes both in strength and in controls (aces and kings, the singleton heart). It would be appropriate to ask for aces with this type of hand. If partner has at least one ace, 6Spade Suitshould have a chance. With two aces, you’d be a heavy favorite. But say your hand was this instead:Spade SuitA K Q 10 5 36 3A 9K Q 7 If you asked for aces, and partner showed one, could you confidently bid 6Spade Suit? If you did, you might catch partner with something like:Spade Suit9 8 7 2Q 10 4K Q 5 4A 6. This will be a silly result if (when) the opponents cash the first two heart tricks. Why was Blackwood not useful here? It’s because of the worthless doubleton heart in opener’s hand. It made 4NT a poor way to try for slam. (A 4cuebid would be better, but that’s a story for a different issue.)
Blackwood isn’t really helpful either if you have a void: Spade SuitK Q J 10 4 A K Q 2 K Q 10 3 You open 1and partner bids 3 (limit). If you bid 4NT (Blackwood), partner shows one ace. Well, which ace is it? If it’s the club ace, slam will be a good shot. If it’s the A, partner’s club holding will be critical to the slam’s success. If he has the Spade SuitA, however, you’ll have two losers on top — the A and theA. You don’t want to be guessing in situations like these, so it’s better not to use Blackwood on these type of hands. As an extension of the ace-asking idea, opener can ask for the number of kings responder holds by bidding 5NT. The responses: 6Club Suit zero or four kings 6 one king 6 two kings 6Spade Suit three kings Again, there’s a catch. 5NT isn’t just king-asking, it’s a grand-slam probe. Responder is permitted to jump to the seven level with the right type of hand. Most partnerships play, therefore, that asking for kings with 5NT guarantees that the partnership holds all the aces. Don’t bid 5NT if (as the Blackwood bidder) you know that your side is missing an ace. Say you held: Spade SuitA 7 6 4 10 2 6 K Q J 6 4 2. Partner opens 1. You bid 1Spade Suit and partner jumps to 3Spade Suit, showing a strong hand. You know that you’re going to bid at least a game, so you cuebid 4 along the way to the spade game. Partner tries 4NT and you show one ace. Partner then bids 5NT. True, you could bid 6, showing one king, but that would be lazy on this particular hand. 5NT guaranteed all the aces, so you know that your clubs will likely provide a huge source of tricks. Bid 7Spade Suit. Note that you can only do this if it’s clear in your partnership that 5NT promises all the aces. Partner’s hand could be Spade SuitK Q J 5 A J 8 6 3 A 5 A 8.