Many years ago, after our side opened the bidding and the opponents overcalled, a double was for penalty. This ineffective method was overturned when Al Roth invented what was first called Sputnik, named after the Russian satellite in the late 1950's. Now, when the opponents overcall, our responder's double is not for penalty. It is "takeout," ("negative") with the focus on the other major.
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There are three crucial issues and two smaller ones. First the big stuff:
1. Through what level?
Many players mark their convention card: "through 2" or "through 3." This is not a good agreement. If you are writing "through 3," do you mean to tell me that when the opponents overcall, say, 4, that you all of a sudden expect to have a trump stack and want to double 4 for penalties? Pretty silly. I recommend negative doubles "through infinity." In fact, the higher they bid, the less likely you are to ever be dealt a penalty double. Doubles on any level should NOT show length/strength in the opponent's suit. They should all be takeout/cards.
2. What suits does the negative double show?
In general, the other major(s) is promised. If responder can't stand a suit that opener selects, he needs to have a backup plan (often tolerance for opener's first suit). If the opening is 1-of-a-major and they overcall the other major, a negative double typically shows both minors. 3. How many points does the negative double show?
This depends on the level. On the one-level, only 6+ points are required. The higher the level, the more points that are needed. On the two-level, about 9-10 is the minimum. The main point is that there is no upper range. A negative double can be made with 13, 17, or even 20+ points!
SPECIAL SEQUENCESThe "smaller" issues are the following two specific auctions, which unfortunately require memorization:
Both majors are promised when the overcall is 1. With only one 4-card major, just bid it. A double guarantees at least 4 cards in both majors.
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When the overcall is 1, a negative double shows exactly 4 spades (no more, no less). A 1 bid would guarantee at least 5 spades.
These 2 auctions cause confusion among inexperienced players. There is no substitute or shortcut for these 2 "special" rules. They simply need to be studied, studied again, then practiced and memorized. If it helps, the only "special" auctions where a negative double GUARANTEES something specific is when the opponents overcall 1 of a RED SUIT.
No other negative doubles ":guarantee" -- but they do tend to show the unbid suits, with the focus on the other major(s).
When playing negative doubles, what if the responder happens to have a penalty double? The way to "get them" is to pass (in tempo is a good idea) and hope the opening bidder can reopen with his own takeout double, to be converted for penalties.
There is plenty more on this topic (such as follow-ups by opener and responder)—but this article is intended as just a brief summary. In my Larry Teaches Doubles book, I go into much more detail (with many practice quizzes). Everyone should play Negative Doubles, and they should be played on all levels (not just the 2- and 3-level). In general, the negative double shows the unbid suits, with the focus on the majors.
You might also consider getting Larry's book on Doubles and Redoubles HERE or try Michael's Negative Doubles Webinar which you can find HERE.