'The most interesting questions from visitors to the Dutch sister site bridgevaria.nl' 

Dear Peter and Ed,

Do you know any defence against weak notrumps of 12-14 HCP, or even 10-12 HCP?

Kind regards,

Willem

Reply Ed Hoogenkamp (South)

Dear Willem,

I myself have no specific ideas about this subject, so I have been searching the web. This is what I found:
 
http://www.cincybridge.com/Lessons/20080312%20Learning%20Points%20Defending%20Against%20a%20Weak%20Notrump.pdf

http://www.marvinfrench.com/p1/biddinghandbook/12-03.pdf

http://www.bridgeaholics.com/bidding/competitive/cant.html

These links present schemes, examples and explanations.

Good luck.

Peter mumbled something like '...penalty doubles...'

Saludos desde Barcelona

Dear Willem,

Ed's links are impressive (unlike his effort in answering...). But an awful lot to memorise...
Those who like to keep it simpler bid the same as over the opponents' strong 1NT (so: Multi Landy, DONT or whatever agreements you and your partner have made).
There is, however, one major difference: be quicker to double (Ed's second link agrees with me). It is a sound principle that a double of a 1NT opening shows at least the same number of points as the upper limit of the strength of the 1NT opening. So you double a 15-17 1NT opening with more than 17 HCP, a 12-14 1NT opening with more than 14 HCP and a 10-12 1NT opening with more than 12 HCP!

But I think that bidding directly over the 1NT opening is not so difficult. The real problems occur when the partner of the 1NT becomes active. So:

¹ 10-12 or 12-14

Many South players let themselves be bullied in this situation. Knowing his partner to be weak, East will, if he is weak as well (but also on many a decent hand) rarely pass 1NT but apply Stayman or a Jacoby-transfer. If he is weak, often the EW bidding ends in ridiculous contracts resulting in many undertricks — but undoubled, since NS don't know how to handle this situation — while NS can make game.

Therefore South has no choice: he has to act at once. He should double East's Stayman or transfer on most hands with the same point count (or better) than the upper limit of the 1NT opening (so he bids as if he is bidding directly over the 1NT opening). In the example above he therefore doubles from 12 or 14 HCP respectively. Such a double is not a lead directing double (which it would be over a strong 1NT opening). The double only shows 'points'.
Isn't such a double risky if East does have a decent hand? Yes, but that can't be helped. It's a consequence of the weak notrump opening. The 1NT opener takes a risk but at the same time he forces the opponents to take risks as well (if they don't dare to do so, they will often be talked out of the bidding). Compare this with a pre-emptive opening (though in that case the risk for the opener is considerably smaller).
Why South has to stir into action at once? Because if he were to do so later, his partner would — correctly — take his bidding as balancing, contesting the part score. So:

¹ 12-14

Here South contests the part score. Since EW have turned out to be (rather) weak, South bids on North's points as well (that is what balancing is about). So if North has 11 HCP he should not bid game, since South has already taken North's strength into account.
If South, with 14 HCP or more, anxiously passes 2♣, NS may miss game (or a juicy penalty), since North, with 10-11 HCP, will — correctly — settle for a part score, since he — correctly — assumes that South has fewer than 14 HCP. The reason? South didn't double at once! If South had doubled at once, North would either have bid game or passed the double.
If the 1NT opening shows 10-12, things are a bit different again, but the principle remains that you should enter the bidding at once if you have a relatively decent hand.

¹ 10-12 or 12-14

Here South bids only his own hand, showing at least 12 or 14 HCP respectively. He can have many different kinds of distribution (he does not show a club suit — he doesn't deny one either — since this is not a lead directing double, as mentioned before). If North has 11 HCP or more, he knows he can bid game or pass the double (if that looks more lucrative). So this time he will not settle for a part score.

To conclude with: the vulnerability plays an important part. As soon as it has become clear that your side has the majority of points, you should consider whether to double the opponents (if that is still possible) or to bid a contract of your own. If the opponents are vulnerable and you are not (brave people, to open a weak no trump in that situation), a penalty double will often be the best choice.
Bidding after the partner of the weak 1NT opener has applied Stayman or Jacoby will rarely be easy. Entering the bidding is risky but you have to take those risks — the weak notrump forces you to do so!