Rule 18: Joinder of Claims

Rule 18 is one of the simplest and most permissive joinder rules. It says that once you are properly in court with at least one valid claim against a party, you may stack on as many additional claims as you have against that same party — even if the claims are completely unrelated. A plaintiff suing for breach of contract can also include an unrelated tort claim against the same defendant.

The catch is that Rule 18 only tells you what is procedurally permissible. It does not create jurisdiction. Each claim must have its own basis for federal subject-matter jurisdiction, or fall within the court's supplemental jurisdiction under 28 U.S.C. § 1367. In practice, unrelated claims joined under Rule 18 rarely qualify for supplemental jurisdiction because they do not share a common nucleus of operative fact with the anchor claim.

Rule 18 applies broadly: plaintiffs can join claims, defendants can join counterclaims, and even third-party plaintiffs can join claims against third-party defendants.