[fromageGB]

After partner's 15-17 NT, You bid 2♣, RHO bids 2♦, passed to you.

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You have no explicit agreement on the meaning of South's pass, but an expectation that it denies a 4 card major.

[whereagles]

3NT. Don't mess up what is simple.

If you do want to mess it up, bid 3♦.

[the hog]

Thrunt.

[PhilKing]

Double. What else?

Yes, partner could have doubled 2♦ for penalties, but he needs a pretty special hand to do so, since I could have been weak.

Double here is still take-out for me (but showing a balance hand), so that's what I do. We win whatever happens, since when partner removes, there is now more room to explore.

[mcphee]

Double appears rather clear cut to me. Partner has no reason to double if front of what can be a poor hand. You have game values but no clear direction. Even if the bidder holds AQJ 6th f D I doubt he will find more than 7 tricks.

[dicklont]

3NT is very dangerous, East may well hold ♦AQJ10x and take 5 tricks from the start.
For some reason East did not open 2♦, most likely because he has only 5 of them.
Do we know this person? Is he a clever tactical bidder who provoces doubles for the win?
Objectively I think double is the best so I do that, but East may have fooled us all.

[Jinksy]

 PhilKing, on 2014-September-02, 04:27, said:

Yes, partner could have doubled 2♦ for penalties, but he needs a pretty special hand to do so, since I could have been weak.

Double here is still take-out for me (but showing a balance hand), so that's what I do. We win whatever happens, since when partner removes, there is now more room to explore.

What's the advantage of giving opener a penalty X at all in this position? For me, if my X is TO (which it is here), P's almost always would be as well.

[Zelandakh]

There are different ways of playing it Jinksy. One popular approach is for double just to show 4 (decent) diamonds, so that pass essentially (almost always) shows clubs. If you do it that way then Responder's double is penalty rather than take out. If partner had the weak hand we were losing more in 2♦X our way than we will defending it. That there are such different methods is one reason I would not double here given the OP conditions. After discussion it might well be the best call though!

[PhilKing]

 Jinksy, on 2014-September-02, 07:22, said:

What's the advantage of giving opener a penalty X at all in this position? For me, if my X is TO (which it is here), P's almost always would be as well.

As it happens, I do play it as takeout as well, but for systemic reasons related to the hands I put through 2♣ - not through choice.

Pen over and takeout under is not ridiculous here - if opener double for take, we are almost never passing. The advantage of the "standard" agreement (pen over takeout under) is that opener double 2♦ with a stack and we still get to defend when he has an OK diamond holding via a reopening double. And responder knows not to double 2♦ holding shortage.

[PhilKing]

 Zelandakh, on 2014-September-02, 08:53, said:

That there are such different methods is one reason I would not double here given the OP conditions. After discussion it might well be the best call though!

But surely that's why virtually all partnerships have general agreements regarding double - you don't need to have them all listed and specifically discussed.