Over 1NT, the bid of a minor by second or fourth hand has these meanings:
2C shows hearts and another suit
2D shows spades and a minor suit
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.