A more thorough explanation:

Definition: Black letter law, also known as hornbook law, refers to standard rules that are widely accepted and not open to debate. These rules are so fundamental that challenging them would be extremely difficult. In English common law, 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: Examples of black letter law include contracts, torts, and land law. These are areas of the law that have been established for hundreds of years and are based on fundamental principles that are widely accepted. For example, in contract law, the principle of offer and acceptance is a black letter law principle that is universally recognized. In tort law, the principle of negligence is a black letter law principle that is widely accepted.

Explanation: Judges often refer to black letter law when dealing with cases that involve these fundamental principles. This is because these principles are so well-established that there is little room for debate. By citing black letter law, judges can quickly establish the legal framework for a case and focus on the specific facts at hand.