3NT Roman Keycard Blackwood
After a Game-force raise :   1♥ - 3♥ - ?   or   1♠ - 3♠ - ?   it is highly unlikely that the Partnership would want to end up in a 3 No Trump contract.   This bid can therefore be use as a 3NT Blackwood or Roman Keycard Blackwood Convention bid instead :

Responses are exactly as after 4NT, only one level lower.   This has the advantage that after a negative response the Partnership can still sign off with a Game bid in a major suit at the 4 level.

Jazclass Recommendation
Whenever a major suit trump fit has been revealed a bid of 3NT by either player is the Blackwood or the RKC Blackwood Convention.

The Blackwood Convention is not very suitable for minor suit trump contracts, because the enquiry bid of 4NT easily pushes the bidding beyond the Game level of 5 without the number of Aces required for a Slam contract.

The use of 3NT is also not possible, because in the sequence 1♦ - 3♦ - 3NT   is to play in NT.

The Gerber 4♣ Convention is not suitable either, for it interferes with conventions like Splinter raises, Cue bidding Aces, Multi Cues, etc.

The solution is to use the 4-level bid of the designated minor trump suit, known as "Minorwood".
For example :   1♦ - 3♦ - 4♦ - ?   or   1♦ - 2♣ - 3♣ - 4♣ - ?   or   1♦ - 3♠ - 4♦ - ?   all are Ace enquiries.
Use the standard 4 RKC steps for your answer :

So that   1♦ - 3♦ - 4♦ - 4♥ shows 0 or 3 keycards   and   1♦ - 3♦ - 4♦ - 4♠ = 1 or 4 keycards

Over Enemy interference use DOPI or ROPI :
            Double (or Redouble) = 0 or 3 keycards     Pass = 1 or 4 keycards

Asking for Kings
Bidding 5 in the minor trump suit is a sign-off. Therefore to ask for Partner's Kings bid the next available bid (but excluding the trump suit) after his Aces reply. For example :   1♦ - 3♦ - 4♦ - 5♣ - 5♥ - ?

Cue bidding is the most obvious method for revealing a void ("first round control") in your hand. But there is another way using Blackwood or RKC Blackwood (whichever method you use).

When holding both one or more Aces and a void, bid your Aces one level higher than you normally would. However use this method only when Partner can identify the void and/or the void will make a positive contribution to the proposed contract.
For example when Partner (who opened the bidding) bids 4NT and you hold the following hand :

Reply at the 6 level : 1♦ - 1♥ - 1♠ - 3♠ - 4NT - you 6♥ -   showing 2 Aces and a void in Clubs (the unbid suit).

Note that by using this method you will miss out the possibility of asking for Kings (with 5NT).
This is where 3NT Blackwood (or RKC Blackwood) have a distinct advantage, as positive void responses are made at the 5 level.

With above hand reply : 1♦ - 1♥ - 1♠ - 3♠ - 3NT - you 5♥ - 5NT (asking for Kings).

(Note : above hand would also qualify for a splinter bid :   1♦ - 1♥ - 1♠ - 4♣ -     showing Spades trump support plus a void in Clubs, see Lesson 26.)

The RKC Gerber 4♣ bid is (like RKC Blackwood) an ask for keycards. Responses are :

A follow on bid of 5♣ is an ask for the Kings (but the trump King is not shown again) :

The Gerber 4♣ bid can be ambiguous (is it Gerber or is it natural ?). It is also incompatible with Cue bidding or MUlti Cues, Splinter raises, Slam Trial bids, etc.

Gerber is therefore not used by top players and is not recommended for students who wish to adopt any of the conventions and methods dealt with in this and following lessons of this course.