A negative double occurs when your partner has opened the bidding with one of a suit and your right hand opponent bids a suit. It does not apply if your partner has opened one no trump or if the opponent overcalls one no trump. After the opponent’s bid, a double by you would promise the two unbid suits, except if only one major has been bid it only promises four cards in the other major. If the auction has gone one of a minor by your partner, minor overcall by right hand opponent, double always promises both majors (four cards in each major). If the auction has gone one of a major by your partner, major overcall by right hand opponent, double always promises both minors (at least four cards in each minor).
The point range varies depending on the level you are forcing partner to bid to.
If partner is able to bid at the one or two level, your double promises 6+ points
If the opponents have pre-empted and partner will have to bid at the three level, your double promises 10+ points
If the opponents have pre-empted and partner will have to bid at the four level, you should have close to an opening hand
Basically though a negative double is an unlimited bid. You are using it to try and find your eight-card fit when the opponents have interfered in your auction.
In the auction 1C/1D by partner, one heart by right hand opponent, Double by you shows 4 spades and 6+ points, 1 Spade by you shows 5+ spades and 6+ points.
In the auction 1C by partner, one diamond by RHO, 1 heart or 1 spade by you shows 4+ cards in your suit and denies 4 cards in the other major.
South's double is a negative double showing at least 6 points and at least 4 hearts.
North bids his second suit, clubs. South corrects to 2♦.
West's double is a negative double showing only 4 spades and at least 6 points.
East bids his second suit, clubs. West passes 2♣.
North's double is a negative double showing at least 6 points and at least 4 hearts.
South bids 2♥ to show that he also has four hearts. North with a minimum hand passes.
East's double is a negative double showing at least 6 points and only 4 spades.
West, with a heart stopper, bids 1NT.
Prepared Using BridgeComposer