copy-paste from http://online.bridgebase.com/doc/gib_system_notes.php

Roman Keycard Blackwood (RKCB)

RKCB is a 4NT bid that, unlike regular Blackwood, asks for “keycards” instead of Aces. There are always 5 keycards - the 4 Aces plus the King of the agreed trump suit. If no trump suit has been clearly agreed, the the King of the most recently bid suit is typically counted as the 5th keycard.

Responses to 4NT RKCB:

• 5C 0 or 3 keycards
• 5D 1 or 4 keycards
• 5H 2 or 5 keycards, but no Queen of the agreed suit
• 5S 2 or 5 keycards plus Queen of the agreed suit
• 5NT An even number of keycards plus an unspecified void
• 6x An odd number of keycards with a void. If 6x is below 6 of the agreed suit then the void is in the suit bid. If 6x is a bid in the agreed suit then the void is in an unspecified higher-ranking suit.

After the 5C and 5D responses, the 4NT bidder can bid the next step that is not a signoff in order to ask for the Queen of the agreed suit. Then:

• Bidding the agreed suit at the cheapest level denies the Queen of the agreed suit.
• Bidding a new suit promises the Queen of the agreed suit plus the King of the suit bid.
• Bidding 5NT promises the Queen of the agreed suit and either denies any side King or denies a side King that can be shown below 6 of the agreed suit.

A subsequent 5NT bid by the 4NT bidder (regardless of whether or not an ask for the Queen of the agreed suit has taken place) asks for specific Kings. The 5NT bid promises that all of the 5 keycards and the Queen of the agreed suit are accounted for. Then:

• The responder to RKCB is entitled to bid a grand slam if he thinks that 13 tricks rate to be available.
• If the responder to RKCB has a King that is lower-ranking than the agreed suit, he should bid that suit at the 6-level. If he has more than one such King, he should bid his lowest-ranking King.
• Otherwise the responder to RKCB should bid 6 of the agreed suit.