The Bluebook: A Uniform System of Citation is a style guide that prescribes the most widely used legal citation system in the United States. It is taught and used at a majority of U.S. law schools, and is also used in a majority of federal courts. There are also several "house" citation styles used by legal publishers in their works. The Bluebook is compiled by the Harvard Law Review Association, the Columbia Law Review, the University of Pennsylvania Law Review, and the Yale Law Journal. Currently, it is in its 21st edition (published July 2020). Its name derives from the cover's color.

The Bluebook
The Bluebook 18th edition cover
SubjectLegal citations
PublisherHarvard Law Review Association, the Columbia Law Review, the University of Pennsylvania Law Review, and the Yale Law Journal

The Supreme Court uses its own unique citation style in its opinions, even though most of the justices and their law clerks obtained their legal education at law schools that use The Bluebook.[1] Furthermore, many state courts have their own citation rules that take precedence over the guide for documents filed with those courts. Some of the local rules are simple modifications to The Bluebook system. Delaware's Supreme Court has promulgated rules of citation for unreported cases markedly different from its standards, and custom in that state as to the citation format of the Delaware Code also differs from it.[2] In other states, the local rules are different from The Bluebook in that they use their own style guides. Attorneys in those states must be able to switch seamlessly between citation styles depending upon whether their work product is intended for a federal or state court. California has allowed citations in Bluebook as well as the state's own style manual,[3] but many practitioners and courts continue recommending the California Style Manual.[4]

An online subscription version of The Bluebook was launched in 2008.[5] A mobile version was launched in 2012 within the rulebook app, an app that allows lawyers, scholars, judges, law students, paralegals, and others involved in the legal profession to reference federal and state court rules, codes, and style manuals on iPad and other mobile devices.[6]