P E N A L T Y   D O U B L E
Partner, My Hand is as Good as His!

Your partner doubles your LHO who has just opened a 15-17 notrump...
    None of this applies if the opening bid is a weak notrump!
Penalty Double
From back in the days of Culbertson and Goren there has been an agreement that when partner doubles the opponent's 1NT bid he is telling you that he has an equal hand...  balanced and 15-18 points.  (Well, in those days it was 16-18 but modern players like to say 15 is enough.)
    What an awful agreement!
Today only a minority of players use a double both as penalty and showing a balanced hand, preferring instead to play a double as showing a single-suited hand or some other feature.  Those who do agree that the double is for penalty usually prefer a long suit as a source of tricks rather than a balanced hand.

Modern Overcall Conventions
Many of our defensive conventions use a double as a way of describing something about the overcaller's holding which is usually a one-suited hand.  A few use an overcall of 2 for that purpose and reserve the double for penalty.  Very few though, have the agreement that the double also shows a balanced hand and those writers who do say it like to advocate that the point-count should start at 17 rather than 15.

Consider the Other Two Hands
Let's assume an average of 16 points in opener's hand and also in the overcaller's hand.  That's 32 points total, leaving your LHO and your partner with 8 points between them.  On average both sides will have about 20 points, right?   So what are your chances of beating the contract?  The guys who like to do the statisitcs for this game will tell you that your chances are about 50-50.  Does that seem like good odds to you?

Partner Doesn't Have His Share
If your partner has fewer than his share of points you will likely give the opponents a good board if he passes, and if he bids you will again likely give the opponents a good board because you will be playing the hand at the 2-level with a very poor dummy... and probably doubled!

Partner Must Bid
A popular suggestion is that the doubler's partner should pull the double with fewer that 5 points, and many player's also suggest playing Systems On - Stayman and Transfer bids.  Another awful agreement!   that means you cannot play in either minor at the 2-level with almost no points!

If you want to plays for tops or bottoms, and never an average, ignore this...
    Give up doubling to show an "equal hand!"         (You can still play penalty doubles, though, if you prefer.)
If you really want to play penalty doubles of the opponent's 1NT call, agree that your partner must pass, even with no points.  Also only double with a source of tricks such as a long suit with outside entries, but never with the balanced hand.   (Be sure to count that you have seven tricks and do not count your partner for even one.  If he has, so much the better, but he probably does not have a trick.)

If you would like to see more on modern agreements on doubling a notrump call for penalty, try:  

Roy Wilson