Voidwood or Exclusion Blackwood (occasionally incorrectly referred to as Lackwood)
These are two different names for the same thing.
Strictly speaking the third name refers to a similar convention which is scarcely seen any more, but the term will occasionally be encountered as an alternative name for the present convention.
There is clearly something of a problem if you wish to use a basic ace-asking convention such as Blackwood or RKCB when holding a void. If you get a response which shows anything other than a full set of key-cards do they fill a gap in one of your suits or do they duplicate your control in the void suit?
Frequently the solution will be to prefer a cue-bidding approach to the key-card-asking convention. The facility to locate specific controls and not just ‘how many’ is a significant advantage of the cue-bidding approach.
But this will not help if you really wanted to use RKCB to locate the trump honours.
Once in a while it won’t matter.
If you hold all of the key-cards outside the void suit you can go ahead and use RKCB anyway. Whether or not partner shows the missing ace you will be able to continue with the queen-ask.
The alternative is to use exclusion Blackwood.
In this convention, once a suit has been agreed a jump to five of a new suit shows a void in that suit and asks for key-cards (or aces if you prefer) not counting any held in that suit.
Post intermediate and above
Context -The continuing auction - in the slam zone - Blackwood etc..
A K J 5 2
A K 6 4
A K 9 8
3♠ is a limit raise.
Your main concern lies with the queen of spades.
Bid 4NT (RKCB). Whether partner responds with 5♣ or 5♦ you will follow with the next suit in search of the trump queen.
K Q J 5 2
A K Q 6
K Q 9 8
3♠ is a limit raise.
All you need to know about are the aces in diamonds and spades.
Bid 5♣ which asks for precisely this information.
Note that a standard Blackwood response showing two controls would not be helpful.
As the exclusion Blackwood asking bid starts at a higher level than RKCB there is something to be said for conserving bidding space with your responses.
Whether you play the traditional 3014 responses or the more common modern alternative of 1430 it is suggested that you should use the lowest response to include the zero key-card response in the present convention.
This convention is strongly recommended as you look to move beyond an intermediate standard.