2NT is the strongest response to 2♢ (whether or not an opponent intervenes). Opener indicates his shortness by bidding the suit BELOW his short suit (whether that shortness be a singleton or a void). However, if his short suit is clubs, it would crowd the auction for responder to ask further with 4♣. Therefore, if opener has short clubs and a void, he shows it directly in response to 2NT. 3NT shows five diamonds, 4♣ shows five hearts, and 4♢ shows five spades (all with a club void).
After opener's rebid of 3♣, 3♢, or 3♡, responder can force to game while asking for a 5-card suit by bidding opener's shortness (next step). (If responder wants to play game in opener's short suit, he bids it!) Opener answers in steps (with 3NT always denying a void). The first step shows five of the lowest possible ranking suit (always clubs), the second step shows five of the next highest possible suit, and the third step shows five of the highest ranking suit. The reason for this structure, is that a long minor is far more likely than a long major (especially spades), and we thereby save space.
After asking for a five-card suit (or getting a response to 2NT of 3♠, 3N, or 4♣), TRANSFER bids, and other special calls apply.
4♣ asks opener about the strength of his hand. Opener answers in steps, with the first step (4♢) showing a minimum, and the third step (4♠) showing a maximum.
4♢ is a transfer to hearts (if opener has shown the suit) or else key card Blackwood for clubs (if opener has shown short hearts).
4♡ is a transfer to spades (if opener has shown spades), or else key card Blackwood for clubs.
4♠ is keycard Blackwood for diamonds (or clubs, if opener has shown short diamonds).
It is virtually impossible for opener to have NO key cards. Therefore, one key card is assumed in the responses when four steps are not available. The first step shows 1, second step 2 without the queen, third step, 2 with the queen. With three key cards we will be in slam, and opener bids the fourth step to show three key cards without the trump queen.
After a transfer to a major, the NEXT STEP is key card Blackwood for that suit, with the standard responses (0 or 3; 1 or 4, etc).
The second step is a short suit slam try in opener's lower side suit, and the third step is shortness in opener's higher side suit.
The fourth step is a 'serious' slam try with no shortness, and the fifth step, 5M, is a mild slam try with no shortness.
NOTE: If opener bids 3♠, 3N, or 4♣ in response to 2NT, the transfer bids apply. You have to be on your toes, or you could end up much higher than you would like, or play in a transfer bid.
If responder jumps to 5m over opener's first rebid, it is a mild try for six, on general values. Doug once reached an 80% slam when he bid 6 on this sequence, and once missed a good 6 when he passed holding just a bit less than a maximum. To sign off in game responder must first ask again.