There is no national citation standard-setting authority, and despite the tendency of citation manuals to attach the word "rule" to specific citation practices, their authoritative reach is, at best, limited to a specific sector those writing for particular journals, editing material for one or another commercial publisher, submitting briefs to a particular court. For most law writing, the relevant citations norms are set by widely accepted professional usage.
The citation manual created by the editors of four law journals, the Columbia Law Review, the Harvard Law Review, the University of Pennsylvania Law Review, and The Yale Law Journal, invariably referred to as The Bluebook, was for decades the most widely used codification of national citation norms. Now in its twentieth edition, The Bluebook governs the citation practices of the majority of U.S. student-edited law journals and has, through its successive editions, shaped the citation education and resulting citation habits of most U.S. lawyers.
The much newer ALWD Guide to Legal Citation (5th ed. 2014) has gained a wide following in U.S. law schools, and since it aims to reflect current usage. Its current edition is highly consistent with The Bluebook.
An earlier competing academic project, The University of Chicago Manual of Legal Citation, which called itself the "Maroon Book," offered a distinctly different and less rigid set of rules. First published in 1989, it failed to win a significant following or affect professional practice except insofar as it recognized the importance of leaving "a fair amount of discretion to practitioners, authors, and editors." Id. at 9.
In some states, the norms set out in national manuals are supplemented or overridden by court rules about the content, composition, and format of legal memoranda and briefs. Most often such rules are largely consistent with national norms but set out special and typically more detailed rules for the citation of cases, statutes, and regulations of the state in question. Some of these state-specific rules call for inclusion of an additional citation element, such as a medium-neutral or other official case citation. Others require less, as, for example, not calling for repetition of the state name or its abbreviation in all state statutory citations, that being supplied by implication. Only a handful of these court rules set out a markedly different citation format. While court-mandated citation rules of this sort formally apply only to documents filed with the specified courts, they are likely to influence professional citation practice within the state more generally.
Courts not only shape local citation norms by their rules governing brief format, their policies for publication and dissemination affect the means of citation. Only a court can effectively establish the means for vendor- and medium-neutral citation of its decisions. Courts that leave the association of an enduring, citable identification for each decision and its parts to a commercial publisher, by default, force the use of the dominant publisher's print citation scheme.
Some courts, including both the Supreme Court and court systems in a number of states, retain full editorial responsibility for citable, final and official versions of their opinions. Generally implemented through a public court reporter's office, this function invariably gives rise to detailed citation norms, as well as other rules of style, that apply to decisions distributed by the court. Where the court's citation format is significantly different from national norms, as it is, for example, in New York, that may or may not influence lawyer citation practice. Courts seriously implementing medium-neutral citation not only attach the necessary decision ID and paragraph numbering to each decision, but use it in citing prior cases.
The large commercial publishers also have their own distinct citation practices, in part designed to reinforce brand. A judge's citation to "Butner v. United States, 440 U.S. 48, 55 (1979)" when reported in West's National Reporter System becomes "Butner v. United States, 440 U.S. 48, 55, 99 S.Ct. 914, 59 L.Ed.2d 136 (1979)." Annotations and summaries in the LexisNexis Lawyers' Edition of Supreme Court decisions cite to the same publisher's United States Code Service e.g., "15 U.S.C.S. § 637(d)."
As noted in the discussion of medium-neutral citation, two important national bodies, the American Bar Association (ABA) and American Association of Law Libraries (AALL), have sought to persuade courts, publishers, and lawyers to implement citation standards that are not keyed to print or to any specific publisher's offerings. The AALL has gone further and published a Universal Citation Guide. This guide sets out a blueprint for courts designing medium-neutral citation schemes for their own decisions, as well as complementary approaches to other types of legal authority that can be implemented simply through professional acceptance. See AALL, Universal Citation Guide (ver. 2.1 2002).
In the end, most of "legal citation," like most of any language, is established by constantly evolving usage, reinforced in some cases, altered in others, by the members of distinct communities.