A bid of 4NT asks partner to respond according to the following schedule:
5C no ace or three aces
5D one ace or four aces
5H two aces of the same color
5S two aces of the same rank (both majors or both minors)
5N two aces of the `odd' combinations
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.