A headnote is a summary which appears before the text of a legal decision, providing information about key points of law brought up in the decision. Headnotes are often added after the fact by editors and publishers, and are not considered part of the decision for legal purposes. When the judge who writes the opinion does write the headnote, it is still not considered part of law; only the decision itself enters into the body of law.
The headnote is designed to act as a quick reference for the reader. For people doing legal research, headnotes are extremely useful because they can be skimmed to determine whether or not the opinion is relevant to the research. For example, if the headnote makes it clear that the opinion pertains to a case about timber rights and the researcher is trying to find murder appeals, the researcher knows that the opinion is not worth reading.
Headnotes can also be useful in the same way that abstracts and summaries are in other contexts, to brief people on what to expect in the text. The headnote can include several keywords along with a brief description of the case and of the points decided by the judge. It also usually includes page numbers which people can consult to see specific areas of the decision. For example, if a headnote states that “the appellant's case was denied on the basis that the lower court's decision was sound,” it will include a page reference to take readers to the section of the opinion which examines the opinion of the lower court and explains why the decision was deemed valid.
Legal headnotes can be useful for people who want quick abstracts for study, as well. Law students are often expected to know case law well, and reading headnotes and taking notes on them can be used to provide the grounding for learning more about the opinion. This is one reason why legal publishers often add them if they are not already present, so that their texts will be more useful to people practicing or studying law.
People who report on the courts often use headnotes as well when they want to quickly report on a legal opinion to make a deadline. They can explore the entire opinion for a more in-depth report at a later juncture, relying on rapid reporting based on the headnote to get the story out before other news outlets.