custom or usage in trade

The proper office of a custom or usage in trade is to ascertain and explain the meaning and intention of the parties to a contract, whether written or in parol, which could not be done without the aid of this extrinsic evidence. It does not go beyond this, and is used as a mode of interpretation on the theory that the parties knew of its existence, and contracted with reference to it. But if it be inconsistent with the contract, or expressly or by necessary implication contradicts it, it cannot be received in evidence to affect it.