A bid of 2C by a passed hand, in response to an opening one of a major, is conventional, showing support for a limit raise of partner's suit asking opener to clarify his range.
Opener rebids 2D if minimum or subminimum. Any other call shows that he has standard strength for opening bid.
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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