The Negative Double Response - Facts

You are East and hold :         ♠ 8 3     ♥ K 9 6 3     ♦ A 7 2     ♣ J 6 5 4
The bidding went like this :

West North East South
1♦ 1♠ You ?  

You are too weak to bid 2♥ and you can not respond with 1NT as that would now promise a stopper in Opponent's ♠ suit.   You must therefore pass.

The Negative Double solves this problem.

West North East South
1♦ 1♠ DBL  
This shows
6-10 points & 4+cards in the unbid major suit(s)
or
11+ points & 4 cards exactly in the unbid major suit(s)
When both major suits have been bid, the Negative Double promises at least 4-4 in the minors.

A special case arises after Opponent's Overcall of 1♥.

West North East South
1♣ 1♥ DBL  
  1. 1♣ - (1♥) - DBL = 6+ points & exactly 4 spades

  2. 1♣ - (1♥) - 1♠ = 6+ points & 5+ spades

Opener's replies
Opener assumes that Partner holds 6-10 points and 4 cards only in the inferred major (or minor) suit(s).
With 4-card trump support Opener replies as if Responder had bid 1 in the inferred suit.
For example :

  1. 1♦ - (1♠) - DBL - (Pass) - 2♥ = 13-15 points & 4-card heart support

  2. 1♦ - (1♠) - DBL - (Pass) - 3♥ = 16-18 points & 4-card heart support

  3. 1♦ - (1♠) - DBL - (Pass) - 4♥ = 19-21 points & 4-card heart support
Knowing that Opener is assuming Responder has 10 points maximum, Responder must respond aggressively when holding 11+ points after Opener's reply.

Without a 4-card trump fit Opener bids naturally, showing next time round delayed 3 cards support for Responder inferred suit.