Black letter law, also known as hornbook law, refers to standard rules that are generally known and free from doubt. The black letter law on any subject consists of the principles so fundamental in that subject and contained so frequently in hundreds of years of common law that challenging them would be extremely difficult.
In English common law specifically, black letter law refers to areas of the law that consist of mainly technical rules as opposed to areas of the law that are defined by a more conceptual basis. Examples of black letter law would be contracts, torts, and land law.
[Last updated in June of 2022 by the Wex Definitions Team]