Control Bidding
If you have a suit in which your side could be off the first two tricks (such as Qxx, or Jx), you shouldn't use Blackwood. For example, Opener holds:
AKJ7642 
AKQ 
32 
3.
You open 1 and partner makes a limit raise to 3. You expect to be in the slam zone. This is not aA 4NT bid to ask for aces (or in RKC, keycards) Blackwoodhand. Let's suppose you useA 4NT bid to ask for aces (or in RKC, keycards) Blackwoodand partner shows 1 ace. Now what? If you jump to 6, you could be facing:
Q1093 J32 Q54 AQ5.
You are in a slam off the ace-king of diamonds--no good. But, what if partner's "one-ace" hand were:
Q1093 Q32 AK64 42?
Now, slam is laydown. So,A 4NT bid to ask for aces (or in RKC, keycards) Blackwoodshould not be used when you have a suit off two quick losers. The solution for these non-Blackwood hands lies in cue-bidding.
“Cue”-Bidding
First, let's explain what a cue-bid is. The term "cue-bid" is a bit misleading. If the opponents open 1 and you bid 2, that is aConventional bid to show a 2-suited hand Michaels“Cue-bid,” showing spades and a minor. This has nothing to do with a "cue-bid" for slam bidding. I prefer to use the modern term, "control-bid" (more specific than the term "cue-bid"). When looking for a slam, we show a control. A "control" is an ace or a king (or, if in a suit contract, a void or a singleton). Think of a control this way: "If we have a control, the opponents cannot take the first 2 tricks in that suit." We use control-bids when we have agreed on a trump suit and are moving towards slam.
Example:
AKJ7642 AKQ 32 3.
You open 1 and partner bids 3 (invitational). We saw above, thatA 4NT bid to ask for aces (or in RKC, keycards) Blackwoodis useless. Instead, you "control-bid" 4, to show that you are interested in slam, and have a club control. In this case, your "control" is a singleton. You know that the opponents cannot cash 2 club tricks. You are worried that they may be able to take the ace-king of diamonds. Your partner will now control-bid 4 for you, if he has a control in that suit. If he has:
Q1093 J32 Q54 AQ5,
he hears your 4 control-bid, and will have nothing to contribute. He will bid 4--he has no control in either red suit. Opener will pass 4.
What if instead, responder held:
Q1093 Q32 AK64 42?
He would control-bid 4 and opener would know there are no suits off the first two tricks. Opener could then useA 4NT bid to ask for aces (or in RKC, keycards) Blackwoodto make sure two aces weren't missing.
When is a new suit a control-bid?
A good rule of thumb is:
Below 3-of-your-major, there are no control-bids. Above 3-of-your-major, once a fit has been found (a suit has been agreed), a new suit is a control-bid (ace, king, void, or singleton).
Example of NOT a control-bid:
1 2
3*
*Not a control-bid. This says nothing about the ace or king (nor length) in clubs. Most partnerships use it as some sort of "naturalish" game try. It is not a slam try.
Example of a control-bid:
1 3
4*
This shows a control in clubs and slam interest (don’t ever control bid without slam interest).
Bid controls in order (whether first or second round). This is the so-called "Italian" method, which I highly recommend. Don't be concerned that you would show a singleton or king before an ace or void--up the line (in order) is the key to making this work!
The trump suit is not a suit in which you make a control bid (RKC will eventually be used to make sure you aren't missing the ace-king of trumps).
Never jump into a control bid.