Black letter law is law that is well established and generally viewed as uncontroversial. In some regions of the world, lawyers may also use this term to refer to case law that has become generally accepted. In legal discussions, when people use this term, they generally do so with the implication that the law in question is accepted and not open to argument, in contrast with other types of laws which may be more open to interpretation.
This legal term is one among many that is often given false origins. People commonly say that it is linked with a famous American text, Black's Law Dictionary, but this is not actually the case. The term “black letter law” was showing up in Supreme Court decisions nearly 50 years before this dictionary was published. The term is more probably a reference to blackletter type, a font family that was commonly used to print legal texts even after it had fallen out of favor elsewhere.
Other terms that people may use to well established law include hornbook law and trite law. “Hornbook law” is a reference to simple, basic, widely accepted law that people learn early in their legal education. In all cases, the terms are used when people are talking about laws that are generally considered quite clear and not nebulous in nature. Many of these laws come from common law, with hundreds of years of history behind them, and people without legal training are often at least vaguely familiar with them.
Since lawyers often question, challenge, and reinterpret the law as part of their work, it is important to distinguish black letter law from other types of law. Unsurprisingly, there is sometimes debate in the legal community about whether or not a law should really be considered uncontroversial. Generally, the older a law is and more frequently it has been upheld in court and in legal treatises, the more likely it is to be evaluated as generally accepted.
Changing societies call for changing laws and differing interpretations on existing legal precedent. This is critical to make a legal system function, as it cannot remain stagnant. However, black letter law often lies at the foundation of the law, and it is used in arguments to support various interpretations of the law. A lawyer might argue, for example, that a reading of related black letter law supports his or her interpretation of a particular statute.