The negative double is a form of take-out double in bridge. It is made by responder after his right-hand opponent overcalls on the first round of bidding, and is used to show both support for the unbid suits as well as some values. It is treated as forcing, but not unconditionally so. In practice, the negative double is sometimes used as a sort of catch-all, made when no other call properly describes responder's hand. Therefore, a partnership might even treat the negative double as a wide ranging call that merely shows some values.
Using the modern negative double convention, it is understood that a double over an initial overcall is conventional, and not for penalties (but see Playing for penalties). For example, using this convention, the following doubles would be regarded as negative, not for penalty:
In understandings regarding negative doubles, the emphasis is on major suit lengths. This is largely due to the special value that tournament play, especially the pairs game, places on major suits. Since the mid-1980s, the negative double has been used mainly to stand in for a bid in an unbid major suit.
Most partnerships using the negative double agree that it applies only through a particular level of overcall. For example, they may agree that the double of an overcall through 3 is negative, and that beyond 3 a double is for penalties.
At rubber bridge many players are reluctant to give up the penalty double of an overcall, and so do not use the double as conventional.
Negative double was the original term for what bridge players now call a take-out double: that is, a double of (most often) the opening bid, intended to show some length in the unbid suits, particularly the unbid major(s).
The term negative double was initially employed to distinguish it from the penalty, or business, or positive double, and signified a double for take-out. Sometime around 1930, the term informatory double replaced negative double, and that term later gave way to take-out double as it is used at present. As terminology changed, the original term "negative" double fell into disuse.
In 1957, Alvin Roth in his partnership with Tobias Stone appropriated the abandoned term negative double to denote a conventional double by responder over an overcall. The bid was also briefly known as Sputnik, because it was as new as the satellite of that name that the Soviet Union had recently launched.
The negative double is generally forcing, but opener might pass to convert the double to a penalty double. There is a special agreement called negative free bids, under which (after the overcall) the bid of a new suit by responder is not forcing. However, most negative doublers play that a new suit response (or free bid), whether at the one level or higher, is forcing.
The negative double loses even more definition when it can be made with a very broad range of strength, from roughly six HCP up to game forcing values. In a pinch, players use it to "get by this round of bidding."
The negative double does not cause the partnership to completely lose the ability to penalize an overcall. There are two ways that the overcall can be doubled for penalties. For example:
Responder makes a negative double, and opener passes for penalties. This position is analogous to one in which a player makes a take-out double and his partner passes the double, converting it to a penalty double.
Responder passes the overcall, opener makes a re-opening double, and responder passes that double for penalties. This can be dangerous, because opener often doesn't know whether responder is simply too weak to make any call, or is hoping that opener can re-open with a double.
These situations are rare, though, and the more so because some five-card major partnerships play negative doubles over minor suit openings only. The rationale is that responder knows much more about opener's distribution after a major suit opening than after a minor suit opening, and can better judge whether to play in opener's major suit, to play for penalties by doubling, or to show a suit of his own.
Partnerships have different understandings about the length in unbid suits that is shown by a negative double, and the understandings differ according both to which suits remain unbid and to the current level of the bidding. Nevertheless, the following are popular understandings: