A Quick and Dirty Primer on Doubles
These are the most common uses of conventional doubles. Volumes have been written on the use of doubles, so don't expect this to be exhaustive. Still, it should clarify most ambiguous doubles.
An ommission is in the area of Cooperative Doubles. Sometimes players refer to them as "Do something smart, partner." These types of doubles are not conventional, nor do any hard and fast rules appear to apply to them.
The best guide I have found is to look at your hand. If the values you have shown with your bids play equally well at offense or defense, then leave it in for penalty. On the other hand, if your values are primarily offensive, pull it. At MPs when the opponents are vulnerable, be more inclined to leave it in as +200 in a part score auction is almost always a top. At IMPs, be more inclined to pull it rather than giving your opponents the chance at a game bonus if you're mistaken.
a. Natural Suit Bid
The double of any natural suit bid through 4D (some play 4H, especially on preempts) is for takeout provided that your partner has not bid or doubled, and the doubler has not made a natural No Trump bid. This is true even if doubler had another bid, is a passed hand, or had made a previous takeout double. An exception is when the doubler had a chance to make a takeout double of that suit earlier and passed.
N E S W
- - 1H P
3H P P X
The double of 3H is for penalty as the doubler could have made a takeout double of 1H.
Takeout doubles are shape showing bids promising at least 4 cards in each unbid suit, especially majors. The word "promise" in this context is the same as in sex. You expect your partner to believe you, but you may have fibbed a little. The less dramatic your shape, the stronger your hand should be to make a takeout X. Similarly, the bidding level and the vulnerability also affect the strength required for the decision. The distribution requirements do not apply to the "Big Takeout Double", see below.
If you are in the balancing seat, the distributional requirements are relaxed somewhat. It still should show a hand unsuitable for a balancing No Trump (12-14 at the 1 level) or overcall. A takeout X in the balancing seat followed by bidding NT at the cheapest level shows a full NT opener which was too strong to bid a balancing NT.
b. No Trump is involved
A double of a 1NT opening or response in the balancing seat is for takeout. It shows a balanced hand, usually 12 to 14 hcp.
A double of a 1NT opening or response in the direct seat is ostensibly for penalty. It usually shows the equivalent of a 1NT opening hand by the doubler. However, this penalty double may be pulled more easily than most, especially if the partner of the penalty doubler has a weak hand with a long suit.
A double by someone who opened 1NT of a suit overcall is for penalty if the overcaller is in the balancing seat and is for takeout if the overcaller is in the direct seat. In the situation where the direct seat makes a penalty double and his partner pulls the penalty double to bid a suit, it is treated as if the direct seat was the overcaller (that's where the points are), and a double by the 1NT opener is for takeout. The opponents have probably found the 1NT opener's small doubleton and his tenaces are of dubious value with the opponent's strength playing over him.
c. The Big Takeout X
A takeout double followed by the bid of a new suit after partner has shown a suit or NT announces the Big X. It shows 18+ hcp and makes no promise of distribution. Partner must erase any thoughts he may have had about the Doubler promising four cards in unbid majors. The Big Doubler is looking for game, and has forced his partner to bid again.
The double of an overcall in a major shows four cards in the unbid major. If 1C is overcalled 1D, the doubler promises at least 4 cards in each major. If both majors have been bid, it promises at least four cards in each minor. Remember what the word "promise" means.
By inference, the bid of a major when a negative double was available shows at least 5 cards in the major. This is important information to communicate to partner. When the auction goes 1C-1H(LHO)-1S(shows 5, else a neg X would be used)-3H(RHO), opener is in a good position to gauge whether to bid 3S over 3H. Slip up by bidding 1S with only four spades, and don't be surprised if your partner makes the wrong choice in a 3 over 3 decision.
An exception to the rule that a mjor suit bid instead of a negative double shows 5+ is when 1C is overcalled with 1D or 2D. Here the negative double would promise both majors, so a bid of a major may be 4 cards only.
The issue of trap passing when playing negative doubles is very important. If a double after opponent's overcall is negative, there is no way for Responder to make a penalty double with a good hand and length in the opponent's suit. So, Responder must pass and trust his partner to reopen the bidding with a balancing double, which Responder can leave in for penalty. A good rule of thumb: if you have opened; overcaller bids a suit you are short in; your partner has passed; and overcaller's partner has passed rather than raising; it's a good chance that your partner has length in overcaller's suit. Bend over backwards to balance with an X.
Negative Doubles were originally called "Sputnik" when they were developed in the early 60s because they were "space age" technology. Some Europeans still use this name. I think it is much cooler name. I also think the negative double to show both minors when both majors have been bid is of dubious value, I'd rather have it be penalty than risk partner passing out your trap pass (for example: a vulnerable opponent makes a weak jump overcall of 2S to your partner's 1H opening, and you hold 10 hcp and KJTx spades). However, no one plays it that way, so be on the look out for trap passes in that situation.
Opponents have bid and raised a suit. Partner has also bid a suit or made a takeout double. In short, the double shows values, no clear place to compete, and suggests partner describe his hand further. If partner has bid a suit, the double shows about 5/5 in each of the two unbid suits. If partner has made a takeout double, then your strength and length is divided among the other 3 suits. These are often great doubles to consider converting to penalty.
4. Support Doubles and Redoubles
You have opened, and partner has bid a major. From the bidding, you do not know if its a 4 or 5 card major. The opponent after your partner intereferes, usually with a takeout double.
Here a redouble of the takeout double (or a double if RHo interferes with a suit bid) shows exactly 3 card support for your partner's major. This way, if he has 5, he knows there is a fit; if he has 4, there is not. By negative inference, if you raise your partner's suit when you could have made a support redouble or double, it shows at least four card support. Again, this inference can be very important if partner is deciding whether to bid 3 over 3.
The use of a double to get your side back in the auction.
The usual rule is that, if your partner has acted at some point, and you acted at your last turn, and your side has not found a fit or bid NT, the double of a RAISED enemy suit is competitive through 2S.
If it is a non-raised suit, it is only a competitive double if a penalty double is impossible from the previous bidding.
Example of a Non-raised Competitive Double
N E S W
- 1D P 1NT
2H P P X
Here, W would have bid 1H if he had 4 or more hearts. So, the double must be competitive. W should have a maximum for his 1NT bid and a desire to compete.
The use of a double at the 3 level, usually when you are an overcaller, to show an invitational raise in competition. The prerequisite is that you have found a fit, and the opponent's interference is in the suit directly below yours.
A bid of your suit would be competitive, not invitational. Since there is no other bid in between their bid and your suit, a double must be used. Usually the overcaller is showing a sound overcall, not light, hence the original name "Maximal Overcall Double."
Example of a Maximal Double
N E S W
1D 1S 2H 2S
Hearts is the suit directly below your agreed suit.
3S would be a competitive raise. The only way to invite game is with a maximal