[Hanoi5]

Generally speaking you will not find opponents willing to intervene to your KC question. However, if they do, people usually claim to play D0P1 or DEPO and apparently that's enough. But it ain't.

I believe most of these counters were designed to treat the interference over the old Ace-only Blackwood. Not many good players continue to use this convention, so I wonder how do you deal with the KC interference.

I know D0P1 is better when they interfere with a suit BELOW your own and DEPO is better if they go over it. Are R0P1 and REPO even needed? Isn't it better to just respond when they double your 4NT or kickback? How about continuations? Is it better to use D as 0-3 and P as 1-4 and then the next step is 2 without the trump queen and then 2 with the Queen? Or just the old double 0, pass, 1, next step 2, next step after that 3, etc?

[fromageGB]

How do you deal with it? Play an ace asking method with sensible replies. Then interference makes no difference to your information exchange, even to showing the Q.

This needs a kickback-similar ace asking bid of the next step up from 4 of the trump suit. Ace ask (5 "aces") is 4T+1. The uninterrupted replies are (using the shorthand of "2+/-Q" meaning 2 aces with or without the Q of trumps) :
step1 = 4T+2 = 0+Q, or 1+/-Q, or 3-Q
step2 = 4T+3 = 2+/-Q
step3 = 4T+4 = 3-Q
step4 = 5T = 0-Q, ie zilch. Simply signoff in trumps. This is always the "minimum" bid.
step5+ = >5T = 4 aces, and this is the king response

Note that this is essentially step responses with steps 1,2,3 for 1,2,3 aces, but modified for the Q of trumps. Easy to remember. Count on your knuckles under the table.

Again without interference, asker's follow-ups are :
Over the step1 = 0+Q, 1+/-Q, 3-Q
. first step = 4T+3 = "I am interested in the 1-ace scenario, as I have 3 or 4 myself"
. . 4T+4 = 1-Q "yes, I have an ace, but no Q"
. . 5T = minimum = 0+Q
. . >5T = "yes, I have the ace you want, I have the Q of trumps as well, and this is my King response"
whereas
. second step = 4T+4 = "I am interested in the 3-ace scenario, as I have 1 or 2 myself"
. . 5T = minimum = "sorry, no 3 aces"
. . >5T = "yes, I have the 3 aces and this is my King response"

Note that this does NOT have the ambiguity of RKCB's "0 or 3" that cannot be resolved.

INTERFERENCE

Assume they bid the step1 suit of 4T+2. Teller simply bids as normal, with X meaning "I would have bid that", ie the step1 reply, and all higher bids are unaltered. Asker continues as normal with no loss of information.

Assume they bid the step2 suit of 4T+3. Teller simply bids as normal, with Pass meaning "I would have bid lower than that", ie the step1 reply, and with X again meaning "I would have bid that", ie the step2 reply. All higher bids are unaltered.

In this latter case, (4T+3) X showing 2 aces, 4T+4 as normal asks for the Q.
In the first case, (4T+3) pass, X from asker is the same meaning "I would have bid that", ie the "1-ace scenario" ask, with the unaltered replies, while 4T+4 from asker is the normal step asking for the "3-ace scenario".

In both these cases of interference we have still conveyed the exact number of aces and the presence or absence of the trump Q.

Assume they interfere by bidding the step3, ie 4T+4 suit, (or 5T-1), then we lose the ability to show the trump Q, but still convey the exact number of aces. Teller forgets about the Q and bids :
Pass = 0 or 3
X = 1
5T = 2
>5T = 4, and is the normal K reply.
There is no ambiguity in the pass for 0 or 3, because asker has a follow-on clarification of X = "is it 0 or 3?" with teller passing or bidding 5T with zero, or giving the normal K response with 3 aces.

If they "interfere" with X, then this is not interference as you just reply as normal.

The interfering bid, in 2 out of the 3 cases, has made absolutely no difference at all. You have full information. Only in the 1 case out of 3, where they bid the suit beneath trumps, have you lost the ability to find the Q, but you still know exact aces.

[Zelandakh]

If you think about it, DOPI and ROPI are essentially the same as 1430 responses from a step point of view. Continuations can follow the same principle of thinking in terms of steps rather than specific calls. Thus after ...4NT - (X) - XX, 5♣ is the queen ask and 5♦ the king ask. This allows the extra space always to be used. This is the same principle that is used by Kickback pairs.