Source: (an extract of) When Should you Double a Weak NT? (Feb/Mar 2009)
Your RHO opens a 12-14 notrump. Holding Kxx/Kxx/Kxxx/Kxx, I think you would pass. Assuming you’re playing penalty doubles of weak notrumps, what are minimum additions you would need to make a penalty double? What are your thoughts on when to make a penalty double vs. making a shape showing bid?
My thoughts on the subject: When an opponent opens a 12-14 notrump the chances of your side making 3NT is slim. I gear my bidding to compete for the partscore and to get distributional games. Therefore I play double shows three types of hands. The most likely hand is a four-card major and a longer minor. My minimum is a hand where I would think about opening. The second type of hand is a minor one-suiter. I could also show a one-suiter by bidding the suit at the three-level. Bidding a suit at the three-level preempts the opponents and makes it more difficult for them to find their best fit. Doubling allows you to play at the two-level if the opponents have the balance of power. Sometimes you can get them on hand types one and two when partner can stand the lead of either minor and passes for penalties. The third type of hand is any strong hand. Over the double, Advancer bids 2
to play in my minor and 2
to play in my major. Two-of-a-major by Advancer shows a six-card or longer suit. If I have a strong hand, I make an unusual bid over 2 or 2. If I have a diamond one-suiter, I pass 2. If I have a club-one suiter, I bid 3 over 2. I lost a tournament when I doubled with a club one-suiter, Responder redoubled and Advancer bid 2 asking for my major. I had to bid 3 which got doubled and went for 300. To avoid this disaster, if Responder makes a strength showing redouble, then pass asks for the minor and 2 asks for the major.
Jeff Rubens—I assume that the first question means “what agreements would you prefer to make about strength to double for penalty.” I prefer very strong penalty doubles. The reason is that if the auction continues responder has a strong clue to his partner’s distribution and advancer does not. It takes a lot of strength to compensate for that disadvantage. For example, one might want to play that advancer’s pass of a two-level rescue is forcing. To support that, doubler needs a very high minimum. Furthermore, how can advancer tell whether to compete with a moderate shapely hand since he has no guarantee of minimum support? If doubler must be very strong, advancer has more protection if he pokes in with a decent suit. There is also a small residual advantage in that having narrower ranges increases the chance of having decent constructive bidding (though it is still very hard in general). On top of all that, if the final contract is 1NT, undoubled or doubled, declarer has a huge advantage. I’d much rather have a good opening lead than a lot of HCP to double 1NT in direct position. A lot of experience as a weak notrumper confirms all these influences.
(2) When not sure, show your shape. Defending 1NT is bad percentage, and if responder has shape (and any strength) he can get an edge various ways if you double. Better not to be involved with a marginal hand.
Jeff Rubens has a lot of experience since he played weak notrumps. The weak notrump side has an advantage. The partner of the opener knows how much strength their side has. Since weak notrumpers know that they will be doubled, they should have good methods to escape to their best fit. Doubling might let the weak notrumper get to a better contract. If you hold 
Qxx
KxxxxJ1098x you would want to play 2 when partner opens 1NT but you can’t. The double allows Responder to play in 2. Another problem is what does Advancer do with four HCP? If doubler has 14, the opponents have the balance of power. If doubler has 18 you have the balance of power.
Eddie Kantar—With a balanced hand: 14+ with a good lead, otherwise 15+. With a strong one-loser suit and at least one certain outside entry or any hand I would have opened. With a two-suiter, show the shape and then bid again to show extras.
One of the problems with doubling with a strong balanced hand such as AxxAxxxAxxKxx, is that you don’t know which is the best suit to lead. Doubling 1NT with the above hand works well when you have a good partner. (A good partner is one who holds the balance of the strength). It doesn’t work well when the notrumper has a good partner who holds the balance of the strength.
Mike Lawrence—My rule of thumb for a penalty double is the equivalent of their max with a GOOD lead or at least one more point. Generally speaking. If I have easily biddable shape, I prefer that route.
Your methods might influence whether you double or bid. If you have AKxxxAKQxxxxx and 2 or 2 show both majors, you could go that way. If 2 shows both majors, partner could easily pass 2 and you would very likely miss a game.
Mel Colchamiro—I would need to add a Queen to one of the suits before I would double. Unless you have great strength, say 18+ HCP, I favor making the shape showing bid, whatever my methods would allow. Isn’t overcalling a weak notrump tantamount to overcalling a 1 or 1 opening bid? Given that, I try to bid as I would under those circumstances. That’s the difference between overcalling a weak notrump and a strong notrump. Their weak notrump is trying to preempt us (and describe), but when they open a strong 1NT(15+), they usually have the balance of power, so we try to razzle-dazzle them, without getting into TOO much trouble (at least at matchpoints).
Frank Stewart—I’d prefer about 15 HCP or so to double. I’d also prefer a potential source of tricks and a reasonable opening lead, but you can’t have everything.