[spil]

1C 1H
1S 2C
3C 3D
What is 3D?

[hrothgar]

spil, on 2021-March-06, 17:28, said:

1C 1H
1S 2C
3C 3D
What is 3D?

Let's see

2♣ was natural and non forcing
3♣ is showing extras, but is not forcing

Partner could have bid 3N, but did not

I would take it as values for game, doubt about NT, inferentially asking about a Diamond stopper.

[morecharac]

spil, on 2021-March-06, 17:28, said:

1C 1H
1S 2C
3C 3D
What is 3D?

Standard American and its offshoots predominate in my area.

Most pairs play it as asking for a diamond stopper; bid 3NT with, rebid something else without it. Or they could be showing 0=4=4=5 shape because they skip diamonds to show a four-card major at the one level and want to let partner choose where to go. Opener probably has something like 4=2=2=5 or 4=2=3=4.

We play it as showing a diamond stopper and preferring partner play the NT game if that's where we belong. It's always felt more natural to me to promise a stopper than to ask for one.

In that auction, if partner can bid 3NT that's where we belong, otherwise I'd aim for 6♣ and stop at 5 if partner shows 0 keycards, one advantage of playing 3014. 6NT might be there but unless there's a feasible way to control cuebid I'll take the chance on a middle board by choosing a suit slam.

[Zelandakh]

hrothgar, on 2021-March-06, 17:41, said:

I would take it as values for game, doubt about NT, inferentially asking about a Diamond stopper.

This much is clear. The question, which varies between partnerships, is whether it asks for a full stopper or only a half-stop. If the latter then you typically need to bid 3♥ or 3♠ when requiring a full stop. As far as I know, there is no absolute international agreement on this distinction. Some pairs still play it as a transfer to 3NT, typically with an anti-positional stopper. I am not sure if any current top pairs use that method but if you look at old books or around the web, you can find plenty of chat about this style.

[DavidKok]

One question on this sequence is what shape(s) opener can have on this auction. I would bypass a four card spade suit with a balanced hand, so 1♠ shows 4(+)♠, 5(+)♣ unless 4=1=4=4 (and some partnerships would open the 4=1=4=4 hand with 1♦, so it is not a consideration). Therefore the 3♣ rebid would promise extra length in the club suit, and denies a good diamond stop by inference (2♦ and 2NT were available, the former showing values and the latter showing a good stopper) - especially since responder can have as few as 3 clubs on this auction. With this much known of opener's hand the 3♦ asks for half a stopper, such as ♦Qx.

Conversely, if the spades cannot be bypassed on the 1-level there is a risk opener is balanced (and might have as few as 3 clubs). Then responder has shown length by giving preference to 2♣ over 1NT, and the raise may be based on something like a 4th club to inform responder that the suit might run. In that scenario 3♦ would merely mean 'my hand just got worth a lot more, but diamonds are a problem', and requests anything from a full to a double stop in the suit.

[paulg]

I keep on changing my mind, which suggests it would be a problem at the table

The mistake is to try and cope with all the hands, since there will be N+1 when there are only N, or fewer, bids available.

It is not realistic for diamonds to be a contract (partner could be 4036 but could have bid two diamonds with this), so you do not need a natural three diamonds here. Also, If you are making a move, then it makes sense that you need at least a half-stop in diamonds.

This lets responder bid 3♥ with a half-stop in diamonds and five fair hearts (where 4♥ is good opposite a doubleton honour).

It leaves 3♦ by responder to show either show a half-stop in diamonds with a suitable hand or a full stop in diamonds with concern about hearts. Opener can continue with three hearts with some support but needing a full stop in diamonds, three spades without help in hearts but help in diamonds, or three no trump with both.

[pescetom]

Zelandakh, on 2021-March-08, 07:13, said:

The question, which varies between partnerships, is whether it asks for a full stopper or only a half-stop. If the latter then you typically need to bid 3♥ or 3♠ when requiring a full stop. As far as I know, there is no absolute international agreement on this distinction.

In Italy the latter is pretty much standard.