A convention... It deals with the problem created for the partner of an opening 1NT bidder following an overcall.
The mechanism varies depending on whether the overcall shows one suit or two and whether it is made at the two level or three level. Over a natural two-level overcall, a double at one time was played for penalty but now is frequently treated as negative.
A two-level suit bid by responder is non-forcing, a three-level suit bid is forcing to game, and a 2NT bid requires opener to rebid 3♣. Responder can pass opener’s 3♣ if he has a weak hand with long clubs or he can bid another suit. If responder bids a suit below the rank of the suit overcalled, it is a signoff; if he bids a suit above the rank of the suit overcalled, it is invitational to game.
A cuebid is Stayman. A relay via 2NT followed by a cuebid is also Stayman. The difference is that one shows a stopper in the opponent’s suit and the other denies it. Direct jumps to 3NT, and 3NT following a relay to 3♣, are similar raises to game without a four-card major and with or without a stopper. It is up to the individual partnerships to decide which sequence shows the stopper and which denies it.
The modern tendency is to play “slow shows” and “fast denies” – slow being the sequence with the 2NT relay, fast being the sequence in which game, usually 3NT, is bid directly.
Over a two-suited overcall, double is penalty oriented in at least one of the suits shown by the overcall. The two level bid of a suit not shown by the overcall is not forcing, while the three level bid of such a suit is forcing to game. Cuebids are generally forcing to game, always at least invitational. Only when the overcall shows two specific suits and responder cuebids the cheaper may the partnership stop below game. Over a three level overcall, the double is a takeout for any suits not shown by the overcall. Suit bids at the three level are forcing to game.
Lebensohl has been modified to extend to other situations.
LEBENSOHL APPLICATIONS. The lebensohl idea can be used, and often is, in two other situations.
(1) Responding to a takeout double of weak two-bids: If a weak 2♠ bid is doubled, a suit response at the three level has an uncomfortably wide range in standard bidding:
West North East South
2♠ Dbl Pass 3♥
North cannot tell whether his partner has 8 HCP or none. With a good hand, he must guess whether to continue to game. Using lebensohl, the responder who bids directly at the three level promises moderate values, perhaps 8-10 HCP or the equivalent. With a weak hand, he must bid 2NT, forcing a 3♣ bid from opener. Responder can pass 3♣ with length in that suit or pick another suit. If the doubler is so strong that he hopes for game opposite a very weak South hand, he can disregard the instruction to bid 3♣.
In an alternate method, 2NT in response to the double asks the doubler to select a suit (any suit after 2♠ doubled, a minor after 2♥ doubled). If the 2NT bidder then bids again (2NT-3♣; 3♥), he shows a better hand than he would have if he had bid 3♥ directly. The same principles apply to second hand after a major suit is opened on his left, raised on his right, and partner doubles. A broadly similar position arises when opener reverses into spades facing a 1NT response: responder’s 2NT can be used as the start of a lebensohl sign-off.
SOURCE: "Conventions", The Official ACBL Encyclopedia of Bridge, 7th ed. Horn Lake: American Contract Bridge League, Inc., 2011, page 295-296. Print