This is part 24 (that means 2 years' worth!) in my tour of the convention card--this series began in January 2008.
Slam-bidding is a subject that can take many books to properly explain. Here, we are just noting on the convention card (and agreeing with partner) our methods for ace-asking.
First, let it be said that Blackwood is probably the most abused convention in bridge. Really, checking for aces should only serve the purpose of making sure you don't reach 6 off 2 aces (or 7 off 1 ace). However, too many people launch into Blackwood prematurely, just because they sense there might be a slam. Blackwood should not be used until you are sure there is no suit where your side is off 2 fast cashing tricks (this often involves control-bidding first). If you use Blackwood and the answer doesn't tell you where you belong, then you probably shouldn't have used it. Don't use Blackwood with a void (see Exclusion).
In regular Blackwood, the responses show 0, 1, 2 or 3 aces in order (5, 5, 5, 5). With 4 aces, the response is also 5, thus 5 shows 0 or 4.
Experienced players use Keycard Blackwood. This means there are 5 keycards--not 4. The king of trumps counts as the 5th "ace." The trump king is presumed to be either a suit that was agreed, or if no agreement was made, then it is the last-bid suit. The answers (as commonly played) are taken from the "Roman" system, thus it is called RKC (Roman Keycard). These are the responses:
5= 1 or 4 of the 5 keycards
5= 0 or 3 of the 5 keycards
5= 2 or 5 of the 5 keycards but no trump queen
5= 2 or 5 of the 5 keycards plus the trump queen
Notice that this schedule is called "1430" because the 5/ responses show 1-4, 3-0. (This is also the score for bidding and making a vulnerable major-suit slam.) When the convention was first devised, the 5 and 5 responses were played the opposite of what is shown here. Many players still use the "old" way, but "1430" is slightly superior because the cheaper 5 response comes up more frequently. After the 5 or 5 response, the Blackwooder can bid the next step to ask for the trump queen (after which the most common system is to play 5 of the trump suit denies the trump queen).
Players using RKC need to discuss what a follow-up bid of 5NT means. Is it for specific kings or number of kings? If the latter, what is the schedule of responses? There is no universal way of playing 5NT; be sure to discuss it with your partner.
There is also a place to mark your responses if the opponents interfere with 4NT. This is so rare, that for most players, I wouldn't recommend bothering with it. If you insist, you can play something like D0P1, (called "DOPI)" which means that Double= 0 or 3 Keycards and Pass = 1 or 4 keycards. Bids show 2 keycards (one step without and two steps with the trump queen).
How to respond to Blackwood with a void? Click here.
5NT (not in a Blackwood auction) --This used to be used as the Grand Slam Force, but a much better and more common use now called Pick-a-Slam. It tells partner, "I want to be in six, but I'm not sure where. You choose."
Gerber--This is one of my pet peeves. Click the link to see my article on Gerber.