Amicus curiae
A friend of the court: a non-party permitted to file a brief offering argument or expertise in a pending case.
An amicus curiae is not a party to the case and seeks no relief. It files a brief because it has an interest in how the question is resolved or expertise the parties may not supply. In the Supreme Court and the courts of appeals, significant cases routinely attract dozens of such briefs from trade associations, states, academics, and advocacy organisations.
They are useful source material for a reason that has little to do with persuasion. Amicus briefs are public, they are frequently the clearest available statement of what an industry or a group of states believes a rule will do to them, and they surface practical consequences that the parties' own briefs skip. They are also a reliable map of who has aligned with whom.
The most consequential amicus is often the federal government itself, filing through the Solicitor General, whose participation signals the executive branch's position on a question it is not otherwise litigating. Every brief filed appears on the case docket, and the resulting decision issues first as a slip opinion.
What the federal government did, once a day.
One email each weekday: the rules, filings, votes and Fed decisions that actually changed something, each one linked to the document it came from.