The lead directing double (LDD) is used to help your partner make the correct opening lead. It is a double by you to show the suit (typically artificial) just bid. At other times, it could be a double of the final contract for a specific lead. Here are the most common uses:
(1) The opponents make an artificial bid (Stayman, Jacoby)
MINIMUM requirements for a two- or three-level LDD after an opponent has opened 1NT or 2NT are: a FIVE-card suit headed by at least three of the top five honors (KJ10xx, AKJxx, etc.) or a SIX-card suit headed by at least two of the top four honors (AJxxxx, KQxxxx). WARNING! Do not make two or three level LDDs with five- or six-card suits that are not headed by the top honors or with shorter suits that are headed by the top honors.
Examples of LDD of Stayman and Jacoby:
WEST NORTH EAST SOUTH
1NT Pass 2
Double
South holds: 
J 8 7 
7 6 
J 4 A Q 10 8 7 6
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WEST NORTH EAST SOUTH
2NT Pass 3 Double
South holds: 4 Q 4 3 2 8 7 6 K Q J 10 6
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WEST NORTH EAST SOUTH
1NT Pass 2 * Double *=Jacoby Transfer
South holds: 4 3 K 4 3 A K J 10 8 J 3 2
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WEST NORTH EAST SOUTH
2NT Pass 3 * Double *=Jacoby Transfer
South holds: 3 K Q 10 8 6 J 7 6 5 4 J 2
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In each case South is making a “PENALTY” double showing both length and strength in the doubled suit.
2) The opponents respond to Blackwood
When the opponents are in a Blackwood sequence (again using artificial responses), you may be able to direct partner's lead with a well-timed LDD.
WEST NORTH EAST SOUTH
1 Pass 2 Pass
3 Pass 4 Pass
4NT Pass 5 ??
South holds: 6 5 J 9 8 6 4 2 3 2 K Q 10
South should double to say he wants a club lead against a potential spade contract. For a double of a high-level artificial bid you don’t have to follow the stricter requirements for lower-level LDD. It is virtually impossible that your opponents will be able to make, nonetheless play, in 5X in the above mentioned sequence.
WEST NORTH EAST SOUTH
1 Pass 3 Pass
4NT Pass 5 ??
South holds: K Q J 3 2 Q 9 8 5 4 2 4 3
Even though you have a six-card diamond suit, you wouldn’t double to request a diamond lead. Let’s say the auction continues:
WEST NORTH EAST SOUTH
1 Pass 3 Pass
4NT Pass 5 Pass
5NT Pass 6
Pass
6 
Pass Pass Pass
If your partner was paying close attention, there is a good chance he will lead a spade, making you happy for once. You didn’t double diamonds or clubs for the lead, so there is a negative inference available. Be aware not only of a LDD by your partner, but failure to make them.
3) The opponents have used 4th suit forcing.
South holds: A Q 4 3 2 5 3 6 5 4 A K J
NORTH SOUTH
1 1
2 ??
If you were South, what would you rebid? 2and 3 are out. Neither rebid is forcing and both show SIX-card suits. You cannot raise to 3 (not forcing) or jump to 4 because raising a SECOND suit promises FOUR cards. In addition, a jump to 4 bypasses 3NT, a possible contract.Bidding notrump yourself with that diamond holding does not look too healthy,so what is left? Right. Bidding the fourth suit. The correct rebid is 2!
As it is VERY common for responder to use the fourth suit TO CREATE A FORCE, the fourth suit is considered to be artificial. Therefore, a double of the 4th suit is a LDD. To double an artificial 4th-suit bid on the 2-level, you have to be careful. The opponents have enough for game and can easily redouble to make you pay. Try to have at least a 5-card suit with 2 of the top 3 honors. Doubling a 3-level 4th-suit GF bid, requires a little bit less, but is still dangerous.
If you would like to learn more about Doubles, you'll find them in Larry's book: Larry Teaches Doubles
Updated: September 2020