What is meant by custom and practice?

Some contract terms that aren't expressly stated may be implied through custom and practice in a particular trade or with a particular employer.

For example it may become customary, if it has become the norm over a significant period of time, to leave early on a Friday in your workplace, or to add an extra day to a Bank Holiday.

Sometimes the idea of custom and practice is used to interpret an express term, for example "reasonable overtime".