Doubles, Support double
Responder, I've got 3 cards in your suit
A support double is made by the opener after his right-hand opponent overcalls partner's response to the opening bid, and is used to say "I've got exactly 3-cards for your suit".
This helps to reduce the chance of getting a 4-3 fit, which is not a great fit at all, of course. The responder can then use this information to decide whether it's best to play in NT for example, if he had proposed a 4-card suit.
A support "redouble" achieves the same goal if there has been an intervening double from the enemy.
With 4 cards, not 3, the opener can bid the suit, of course.
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