Used by opener to show exactly 3 trump support for responder's suit, whereas a direct raise shows 4 trump support.
S: xxxx H: Axxx D: Qxx C: Qx S: x H: KQx D: KJxx C: Axxxx W N E S 1C P 1H 1S X P 2H P/O
Notes: South's double is a support (raise) double, showing exactly 3 trumps. If South bid 2H, he would show 4 trumps.
A bidding convention is an agreement between two players on a different meaning of a bid in the card game bridge. These agreements are made so that partners can more quickly determine which contract, with both their cards, they can win together. Often, conventions are forged together into one, which is then called a bidding system.
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Bridge Conventions (List)
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