[vodkagirl]

Playing Roman Keycard Blackwood, after 4N, 5N shows 2/4 keycards and an unspecified useful void.

Questions:

1. Is a "useful" void a suit that we have not bid?

2. How do I discover what the void is?

I have been going thru some of the online material and these two questions do not seem to be answered by it.

[wank]

the assumption is it's obvious - you'll normally have splintered or it'll be a suit the opps have been bidding.

[FrancesHinden]

1. A 'useful' void is one that isn't in partner's suit.
2. If it's not going to be obvious to partner what suit the void is in, don't show it.

In general, you need to be careful about showing a void because it forces to slam, so you have to be certain that you have enough keycards for slam. You can't count the void as a keycard for that purpose because partner might have the ace anyway.
Compare and contrast these two auctions:

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AQxx
AJxxx
Jxxx

you deal and open 1D, partner bids 1H and decide to raise to 3H. Partner jumps to 4NT, keycard for hearts. Now it's possible that your spade void is useful, but you shouldn't show it: you only have 2 keycards, you have a made a light opening bid and it's quite possible partner has a strong hand like AKx KJxxxx xx Ax. Basically you've already 'counted' the void in your high cards when you chose to raise to 3H.

Now you have
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AQxx
AJxxx
Jxxx

LHO opens 2S, partner doubles, RHO bids 3S and you bid a (slightly conservative) 4H. Partner bids 4NT, keycard for hearts. Now you would definitely show the void: it's easy to see it must be right because your usual response (5S) would force to slam anyway but even without the HQ you have a huge hand and it's very likely the spade void is good news for partner.

[ColdCrayon]

Just piping in here - according to Eddie Kantar who's as close to the authority on RKC as anyone, you should know where the void is. It's also obligatory; with 0 keycards (technically an even number) whether or not to show the void is dependent on partner agreement, situational factors, etc., but with 2 keycards and a useful void, 5NT is mandatory; if you fail to make that bid you're misleading your partner, which is a cardinal sin when they've assumed the captaincy.

If you don't like that system, come up with another - you can make 6c an asking bid, where responder will show the void suit.

[vodkagirl]

Let us say, partenr shows a useful void with 5N.

I am jumping to 7 in our suit? or am I staying at 6 if I can't account for the trump Queen?

[Mbodell]

Imagine the following hand (rotated):

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You've got a control rich 16 count and 2 small in partners suit that he's opened a strong 2 clubs with!

One approach, likely taken by about 10% of the field in the finals of the NABC+ Fast Pairs might be just to bid 7nt. If instead you bid 4nt over 3♠ you'd hear a 5nt response from partner. Now it is extremely likely that partner is void in one of your good suits. But something like AKQ-eighth, stiff, void, QJ-fourth might be a 3 loser distributional 2♣ opener, so it isn't certain. You'd like to check for partner's void and bid 7♠ if it is diamonds and likely 6nt if it isn't.

If you try 7nt you'll get doubled and the ♦A is the opening lead. I sadly know that -100 gets you 4 out of 64 matchpoints. Here's the full hand (at our table over 3♠ it went 7nt X -1. I've included funny fake play to make 7♠ not only after a favorable heart lead, but also if you get a sloppy trump play from both defenders as opening lead you have a cute squeeze and trump transport on T2):

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