What is a negative double?

Negative doubles are a kind of take-out double. We use them when opposition has overcalled. For example if partner opens 1♦ and opposition overcall 1♥ and you double, this would be a negative double.

What do they show?

The most important thing that negative doubles show is a four card suit in the unbid major. 

You need some points for a negative double but not as many as a usual take-out double. It all depends on the level to which you are forcing partner to bid.

If partner still has an available bid at the 1 level then you can make a negative double with 6 or 7 points. E.g. 1♦ – 1♥ – x  still leaves partner the opportunity to bid spades at the 1 level. 

If you are forcing partner to respond at the 2 level then you need 8 or 9 points. E.g. 1♦ – 1♠ – x means that partner must bid at the 2 level if they want to bid a suit.

If partner has to bid at the 3 level then you need 10 points plus. E.g. 1♦ – 2♠ – x

Here are a couple of hands which would make a negative double.

If you have 5 cards in the unbid major then you should usually just overcall naturally.

How should opener respond to a negative double?

If you can support the four card major indicated by responder’s negative double then do so. With a hand worth 12-15 raise 1 level. If you are worth 16+ jump bid.

If you cannot support partners major, but you have a second suit you were planning on bidding then do so.

If you have a balanced hand with at least 1 stop in opponents suit then you can bid NT. 1NT with 15-16 points, 2NT with 17-18 points and 3NT with 19.

If you cannot make any of these bids, but opened with a long minor suit then repeat your suit.