Negative Doubles are used primarily after your partner has opened and your RHO overcalls.

The most frequent use of the negative double is to show a four card holding in one or more unbid major.  **However, a negative double also is correct with five or more cards in a major when the hand is not strong enough to bid the suit directly. 

In the examples below we assume that a new suit at the 2 level in a constructive auction, shows 10+ HCPs.

Use of Negative Doubles

Partner	RHO	You
*1	1	X	=	6+ HCP and four spades . A bid of 1 promises five spades
**1	1	X	=	6+ HCP and four hearts or five with insufficient pts. to bid 2 naturally
1	1	2	=	10+ HCP and five hearts (as it would without the overcall)
1	2	X	=	6+ and four spades. Can stand partner rebidding 2  A rebid of 2 by partner would NOT be showing reversing values.
1	2	X	=	6+ and four hearts. Can stand partner rebidding 2
1	1	X	=	6+ and both minors. Can stand partner rebidding 2
1	x	X X	=	6+ HCP and at least 1 four card major. A bid of 1 or 1 shows five.
1	x	X X	=	6+ and four spades. Can stand partner rebidding 2
1	2	2	=	Natural, 8+ but NOT four spades. Can stand partner rebidding 2  Any response of a minor denies four of a major.

* The beauty of negative doubles is that you can now use the interference by opponents to add more clarity to your auction.  e.g. 1 - P - 1 Your responding bid of 1 could be four or five spades.

After 1 - 1 - X shows four spades.         1 - 1 - 1 shows five!