A hornbook law is a basic legal principle that is so well-established and widely accepted by the courts and the public that it does not require further explanation or clarification. Legal principles considered to be hornbook law have been tested through case law so many times in the past that they are now accepted without further explanation. Many hornbook principles are derived from common law and are backed by hundreds of years of case history. As such, they are completely unambiguous and beyond dispute today.
These basic principles are also sometimes referred to as black letter law or trite law. Law students learn hornbook law early on in their academic careers. Knowledge of these principles is necessary for the analysis of legal cases, which makes up a bulk of their study load.
The term "hornbook" was originally used only in reference to a text that served as a tool for study. The phrase has been used for different types of study aids in varying fields before they took their current form as textbooks. The name came into use as it referred to a page covered with horn that was thin and transparent. The page was fixed into a frame with a handle on the side so that the reader could hold it up to read without having to touch the horn.
In the law, a hornbook is a textbook that provides a primer on basic legal principles, or hornbook law. These books explain these fundamentals legal principles in an easy-to-read narrative. They condense a specific area of law into a single volume, providing a history of how the law evolved and an explanation of how it is applied today by the courts.
Hornbooks are somewhat less detailed than treatises and include fewer references to other sources, making for smoother reading. They do, however, provide citations to relevant cases so that law students can see how the hornbook laws were applied in real life. They are also good at explaining the finer points of the law because while providing less detail than treatises, they do offer a high level of detail.
While hornbooks are a great way to learn law subject by subject, they are used more as a study aid than as primary course reading. Law students often use them in conjunction with the required case law readings, because hornbooks can help them understand the basic legal principles on which cases were built.