Custom

To be a source of law, a local custom had to meet certain criteria.
It had to be

• clear, certain, and consistent
• reasonable
• obligatory and exercised as of right
• geographically local
• in conformity with any applicable statutes (otherwise these overrode the custom)
• continuously held since "time immemorial"

Time immemorial, per se, meant a time antedating any legal history. However, for clarity, this was taken by reference to a particular voyage of the King of England to France. In 1275/6, again to avoid compication, this was finally fixed to have been at the beginning of the reign of Richard I in 1189.
This had the additional effect that a claimed custom could thereafter be defeated simply by showing that its exercise would hve been impossible at that time -- for example a claimed customary right of grazing on land which, on enquiry, proved to have been underwater at or since 1189.