No matter what you’re writing, the purpose of a citation is to allow the reader – judge, partner, professor – to find the law you’ve researched and compiled. The citation needs to be correct with all the relevent pieces of information, that's why citation format is important. The legal community uses The Bluebook: A Uniform System of Citation format rules. The Bluebook is now in its 20th edition, published in 2015. Also many courts have their own local citation rules that supplement or even replace The Bluebook rules. A correct citation will:
Court opinions and articles don’t change once published. There can be more recent articles on a subject. An author can change their mind about a topic but their orginal article remains as initially published. Court opinions can be effected by later decisions, their authoritative value can be reduced or eliminated but the text of the original opinion will remain the same.
We can take an opinion, stick it in a book, and just leave it there. But that’s not true for statutes and regulations. Every time a legislature meets some statute is going to change and when statutes change then the regulations change. Because of that dynamic character we don’t just stick statutes and regulations in books and leave them there. The books are updated regularly. So the citations to statutes and regulations have different numbering systems.
4 U.S.C. §553
The U.S Code is a codification. That means it is a subject arrangement of all the U.S. statutes. There are 51 subjects which are referred to as Titles. A section number is not necessarily unique so the title number is necessary.
There are official and unofficial versions of the U.S. Code. The government publishes the United States Code. The other two versions are publications of commercial companies but you may come across references to these codes in your reading.
United States Code Annotated (USCA)
United States Code Service (USCS)
The reason people buy the commercial (also known as unofficial) versions of the code is because of the editorial enhancements. These enhancements, also referred to as annotations, help a researcher understand the meaning or application of a particular statute. If you already have a citation to a pertinent statute you can use it to help you research an issue.
The citation above is to the Administrative Procedures Act, specifically the section on rule making (P.L. 79-404).
Below are examples of how to read code, public law, and Statutes at Large citations:
Legal journals or law reviews have a unique citation format dictated by Bluebook rules.
79 U. Cin. L. Rev. 375 (2010)
The Gallagher Law Library at the University of Washington has a nice table of abbreviations of law reviews and legal periodicals.
The citation is to an article entitled "Preserving the Right to a Jury Trial in Public Employee Free Speech Litigation: The Protected Status of Speech Must be Labeled a Mixed Question of Law and Fact". Titles to law review articles can be very long and descriptive.
483 U.S. 378, 107 S. Ct. 2891 (1987)
There are actually two citations here, The second with "S.Ct." referrs to another reporter Supreme Court Reports. They both contain the opinions of the U.S. Supreme Court. One is published by the government (U.S.) the other by a commercial publisher. This use of multiple citations to the same court opinion is referred to as parallel citation. The citation is to Rankin v. McPherson, a case about free speech rights of public employees.
Sometimes courts require attorneys to include the parallel cite in documents filed with the court. Most courts have adopted The Bluebook: A Uniform System of Citation (we just say Bluebook) but some have adopted their own rules, some specify older Bluebook editions. Following the court rules on citation format is extremely important.