A penalty double should be used if your opponents contract is likely to fail. A successful double increases the penalty and can be a effective route to good scoring. A double can be used where you have a good holding of trumps, your opponents have overbid their strength or you have a long suit against a NT contract. Where your opponents have bid at the 5 level, against your bidding, using a double should always be considered.
Doubling low level contracts is risky. Getting 7 or 8 tricks is often not difficult. For this reason the double is used for other purposes on bids less than 3C.
The only exception to this rule is when 1NT is doubled. In this case the double is always for penalty and shows 16 plus points. When 1NT is doubled the responder must judge whether the contract will succeed. The responder adds her points to 12. If the total is less than 19 the contract is likely to fail and the responder bids her longest suit. The opener must not rebid.
For the doubler’s partner the same logic applies. For the double to be safe, 21 points are required. Adding your points to 16 determines whether evasive action is required. If so bid your longest suit.
When opposition have opened, you can find an overcall difficult to find even though you may have the strength. By using the takeout double you are showing an opening hand with at least 3 cards in each of the unbid suits. A takeout double obliges your partner to bid her longest suit irrespective of her hand’s strength. This bid is not mandatory if the opponents make an intervening bid.
After an intervention a response to the takeout double can still be made by a free bid. This shows a hand of sufficient strength and/or length to be worthwhile.
NB When an opening bid in a suit is doubled it is never for penalty and obliges partner to bid.
When your partner has opened and your opponents have overcalled, it is often difficult to respond because of no five card suit and insufficient strength to bid at the 2 level. The negative double shows 4 cards in the major suit not bid by the opposition i.e. 4S if hearts bid by overcaller or 4 hearts if spades bid by overcaller.
The opener must bid again unless there is an intervention.
When your opponents are using an artificial bid such as a Red Suit Transfer or Stayman, a double can be used to show strength in the bid suit.
For example - 1NT - PASS - 2D(transfer) - double (strength in diamonds).
The double will not be left in by the opener because the responder has hearts not diamonds. This useful information can be used to enhance the defence in the choice of lead.
A lead indicator double does not indicate overall strength or a desire to compete in the auction.