In bridge bidding, a Snapdragon double is a competitive double after the first three players bid three different suits. A Snapdragon double by the fourth player is artificial and forcing. It advertises length in the lone unbid suit (5+ cards), at least 8 points, and tolerance for partner's suit. "Tolerance" ranges from doubleton to tripleton support depending on partnership agreement. The following examples will contain middle-of-the-round support, i.e. honor doubletons like A-x or K-x.
South's double shows a spade suit with tolerance for diamonds. The reasoning behind the Snapdragon double is that a natural penalty double here is very impractical. Snapdragon nurtures the hope of playing in either of two suits.
Vulnerability and bidding level are factors in determining whether to double, and Snapdragon is typically played through the 2-level. At the 2-level, however, it promises a better suit and values:
South's double indicates a good heart suit with spade tolerance. Now imagine the following auction:
South should pass with flat shape and anemic clubs that would require getting up to the 3-level.
By inference, bidding the fourth suit instead of doubling denies
support for partner:
denies club tolerance because he failed to make a Snapdragon double.
The Snapdragon double is forcing for one round. In response to a Snapdragon double, overcaller's rebids are natural.