US legal citations tell you where to look to find a legal document, based on the volume and page number where the case may be found in a series of books called reporters.
Cases are found in multiple reporters (official and unofficial), so you may encounter multiple different citations for the same case. These may be referred to as "parallel citations," that is, parallel to the official citation of the case in United States Reports (the official reporter for Supreme Court cases) or the official state reporter.
Case: New York Times Company v. Tasini
533 U.S. 483
this is the official citation for this case; it refers to the case as published in United States Reports
121 S.Ct. 2381
this is an unofficial citation for this case; it refers to the case as published in Supreme Court Reporter
150 L.Ed.2d 500
this is an unofficial citation for this case; it refers to the case as published in United States Supreme Court Reports - Lawyers' Edition
(Adapted from Georgetown Law Library Guide to Citing Cases)
When citing a Supreme Court case, you should cite the official Supreme Court reporter, United States Reports, unless the United States Reports volume containing the case has not yet been published. If the case you're citing has not yet been published, cite the United States Supreme Court Reports - Lawyers' Edition.
As of March 2014, cases decided on or before October 3, 2008 are available in United States Reports.
1. U.S. Supreme Court: Official Citation
When citing Supreme Court cases, you must cite to the official Supreme Court reporter, United States Reports. To cite to a case in the United States Reports, list the following five elements in order:
Consider, for example, the following citation:
New York Times Co. v. Tasini, 533 U.S. 483 (2001)
The elements are as follows:
|Name of the case||New York Times Co. v. Tasini,|
|Volume of the Reporter||533|
|First page of case||483|
|Year of decision||(2001)
If you're referring to a specific page of a case, add the page number(s) as shown in the example below.
2. United States Supreme Court Reports - Lawyers' Edition: Unofficial Citation
When the case you're citing has not yet been published in the official United States Reports, cite the United States Supreme Court Reports — Lawyer's Edition instead. Citation formatting is the same, except the reporter abbreviation differs. For example:
Reed Elsevier, Inc. v. Muchnick, 176 L.Ed. 2d 18 (2010)
When reading law review articles and other materials discussing Supreme Court cases, you may come across references to cases. Knowing the abbreivations for the major Supreme Court reporters will make it easier to decipher the citations and find the relevant opinions.
U.S. United States Reports (official)
L. Ed. United States Supreme Court Reports, Lawyers Edition (unofficial, LexisNexis Publishing)
L. Ed. 2d --- Series 2
S. Ct. Supreme Court Reporter (unofficial, West Publishing)