[cencio]

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[DavidKok]

Over RKC Blackwood, 5NT commonly shows an even number of key cards with a useful side void, while 6-of-a-suit below the trump suit shows an odd number of key cards with a void in the suit bid. If your void is in a higher ranking suit, bid 6 of the trump suit instead.

[mike777]

I would add that many do not play RKC over
The auction that starts
1X= 4NT

I was taught that you simply respond with the number of actual aces.

Here I would respond 5D, showing one ace.

Granted this is an extremely rare auction.

[pescetom]

I thought I already explained you all this here

[cencio]

pescetom, on 2026-August-03, 16:37, said:

I thought I already explained you all this here

Repetita iuvant 😀

[pescetom]

 DavidKok, on 2026-August-03, 11:33, said:

Over RKC Blackwood, 5NT commonly shows an even number of key cards with a useful side void, while 6-of-a-suit below the trump suit shows an odd number of key cards with a void in the suit bid. If your void is in a higher ranking suit, bid 6 of the trump suit instead.

I also play that following either of these bids the next step asks for Q of trumps, with a Specific King as positive response.
Although I have seen it recommended to use 6♣ over 5N as an ask for the void suit instead.
Of course if trumps are not spades, you may not have room for such subtleties, as is often the case in RKCB.