The Negative Double is very similar to a takeout double, except that it is bid by the responder.    It is used in a competitive auction to shows a hand that either has insufficient strength or length to make a natural bid.              (Note: A suit bid tends to show 5+ cards and a bid at the 2 level should show 10+ points)

Partner opens the bidding on the 1 level and the opponents make a direct suit overcall.  Since a penalty double at this level would not be practical -- a double by responder is used for takeout.  The purpose of the negative double is to find a 4-4 fit with emphasis on the majors. The "negative" double shows "values" and a willingness to further compete.  

Strength Requirements

6 Points on the 1 level

8 Points on the 2 level

10 Points on the 3 level

Negative Doubles are quite flexible.  If a major suit has been bid -- a negative double promises the other major.  If both minor suits have been bid on the one level -- you promise both majors.   Since you promise both majors for your double -- a bid of a major suit on the one level only promises 4 pieces. 

  If both major suits have been bid -- you promise satisfactory values in the minors.  If responder does have a decent long suit but doesn't have the HCP's to introduce it on the 2 level or above -- he should start with a "negative double"  and then bid it on his next turn.  A double and bid of a new suit by responder shows a long suit and a weak hand.

The level to which you play negative doubles is subject to partnership agreement.  I generally play them to 4D -- giving special consideration to preemptive bidding by the opponents. 

How do we double for penalty?  Sometimes responder does have a decent hand with extra length and strength in the overcaller's suit.   Responder must "pass smoothly" and wait for his partner to re-open the bidding with a double which can be left in for penalty.