An unwritten law that gained its validity through long use. General Custom (Common Law) - unwritten law that has developed over the years based on the content of judicial decisions. This is in opposition to statute law; law enacted by Parliament. Particular Custom - The customs of a particular district that affect only members of that district. One of the ways in which a practice or benefit can constitute an implied term of a contract of employment.
A practice or benefit will become an implied term of the employment contract if it is regularly adopted and is customary in a particular trade or locality or at a particular workplace.
For the custom or practice to amount to an implied term of the employment contract, it must be "reasonable, certain and notorious" (Bond and another v CAV Ltd  IRLR 360) and followed "because there is a sense of legal obligation to do so" (Solectron Scotland Ltd v Roper  IRLR 40).