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.
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? You'll have to come to one of my lectures to ask me that one :)
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.
Larry's Audio Tour of the Convention Card
For descriptions of most conventions mentioned in this article, see the ACBL page on Commonly Used Conventions.