A more thorough explanation:

Definition: Hornbook law, also known as blackletter law, refers to legal principles or concepts that have been established and accepted as part of the law for a long time. These principles are difficult to change because they have been affirmed in cases for many years. The term "hornbook" comes from the books used to teach law students the fundamental concepts of an area of law.

For example, the concept of "innocent until proven guilty" is a hornbook law. This principle has been a fundamental part of the legal system for centuries and is deeply ingrained in the law. Another example is the principle of "res ipsa loquitur," which means "the thing speaks for itself." This principle is used in cases where the facts speak for themselves and no further explanation is needed.

Hornbook law is important because it provides a foundation for the legal system and ensures consistency in the application of the law. Because these principles have been established for so long, they are unlikely to change unless there is a significant shift in societal values or legal philosophy.