Once you start reading cases in American law, you'll eventually notice that
United States Supreme Court cases are cited another way, too. Before the
1875 term, the United States Reports were numbered and cited based on who was the official
Reporter of Opinions of the Court—that is, who was actually writing down and typesetting the lawyers' arguments and the justices' opinions.
Marbury v. Madison, for instance, is cited as 5 U.S. 137 under the current system, but would have been cited as 1 Cranch 137 under the old system, as the fifth volume of United States Reports was reported by
William Cranch, and it was his first volume.
Some lawyers, judges, and legal scholars still use the old system alongside the new system: Marbury would thus be cited 5 U.S. (1 Cranch) 137, or 5 U.S. (Cranch) 137. The old system is never used on its own nowadays, and for most lawyers, it's easier just to skip it (like they skip citations to the Supreme Court Reporter and Lawyer's Cooperative Edition).
A list of the reporters, along with their abbreviation, term, and the volumes they reported:
Bound copies of these volumes have both the old and the new volume numbers written on the spine.