For legal purposes, a custom may be relied upon to provide an implied term of the contract or else to assist in interpreting any unclear terms in a contract. The use of custom to establish or interpret a term of the contract is more common in commercial, especially shipping, cases. There are very few employment cases to refer to as illustrations. But it has been decided that the custom of paying non-unionists a lower wage than unionists was unreasonable, and that even though an individual employee was unaware at the time he was hired of the custom of making deductions for bad workmanship in the Lancashire weaving trade, the custom was nonetheless "notorious" (i.e. well-known in the locality or particular industry).
If the parties have included an express or implied term in the contract, then it is not permissible to prove a custom inconsistent with the term.