The support double is a double used by opener after partner makes a suit response and the next player interferes. The support double is not penalty-oriented, but shows exactly 3-card support for partner and unlimited strength. A direct raise, consequently, shows 4-card support, while any other action denies support. Examples:
Shows exactly 3 hearts.
Shows 4 hearts.
Shows 6+ hearts and denies as many as 3 spades. Making a support double is opener's first priority.
Responder's rebids are natural.
If the opponents' interference is in the form of a double, a redouble by opener can be used to show 3-card support. This of course is called a support redouble.
Support doubles are on through all competition up to and including 2.