[McBruce]

A pair has an uncontested auction where responder's rebid of 2C is the fourth suit. They play fourth-suit forcing but there is no alert. Opener continues with 2NT and responder raises to 3NT. A face-down lead is made (not yet exposed) and responder/dummy now says that there was a failure to alert the 2C 4SF call. Opening leader wants to know whether the defenders have the right to know whether this is a forgot-to-alert situation (meaning declarer will have a club stopper) or whether this is a forgot-our-agreements situation (meaning declarer may not have a club stopper). The actual question: "when you bid 2NT, were you aware that 2C was artificial?"

What Law or ACBL regulation makes this question legal or illegal?

[pran]

McBruce, on 2013-August-08, 05:14, said:

A pair has an uncontested auction where responder's rebid of 2C is the fourth suit. They play fourth-suit forcing but there is no alert. Opener continues with 2NT and responder raises to 3NT. A face-down lead is made (not yet exposed) and responder/dummy now says that there was a failure to alert the 2C 4SF call. Opening leader wants to know whether the defenders have the right to know whether this is a forgot-to-alert situation (meaning declarer will have a club stopper) or whether this is a forgot-our-agreements situation (meaning declarer may not have a club stopper). The actual question: "when you bid 2NT, were you aware that 2C was artificial?"

What Law or ACBL regulation makes this question legal or illegal?

Opening leader (and his partner) has the right to know the correct partnership understanding on the 2♣ bid.

They have no right to know whether it is a forgot-to-alert or a forgot-our-agreements situation, but they are at liberty to infer this from the correct explanation of the 2♣ bid. However, such inference will in case be made on their own responsibility.

[blackshoe]

Law 75C says, inter alia, they [East-West] have no claim to an accurate description of the North–South hands.

[mycroft]

They are entitled to the agreements. This includes partnership experience with 4SF calls, in particular whether they've been forgotten in the past; they're also entitled to partnership experience with "partner knows our agreements, but forgets/doesn't realize they have to Alert". With that information, they can decide what's likely to have happened. I see nothing in the Laws that requires the opponents to answer the question in the OP, however - I'm not required to tell them what I thought about anything, never mind the 2♣ call.

Responder, of course, can't use the failure to Alert in her calls.