The following excerpt is from a recent article posted at www.masteringbridge.com:
In the world of old-fashioned Standard American, if you are not playing Negative Doubles, then the following Auction
would have to be a penalty double. This in effect dares the opponents to make their contract of 1 Spade, suggesting that they cannot make it.
Since the opportunity to double the opponents for penalties at this low a level comes up so seldom, the word “double” is now used to have a different meaning, in the world of modern bridge. Thus, the Negative Double was invented.
Imagine that you pick up this hand:
Partner opens with 1♦. Your RHO overcalls 1♠. If you don’t play negative doubles, you would have to pass. Playing negative doubles, you will now say “double” & this will show partner that you have the 2 unbid suits AND enough points to respond to partner’s opening bid (6 or more points)