Rule 20: Permissive Joinder of Parties

Rule 20 governs when multiple plaintiffs can sue together or multiple defendants can be sued together in the same lawsuit. Both prongs of a two-part test must be satisfied: first, the claims must arise out of the same transaction, occurrence, or series of transactions or occurrences; second, there must be at least one common question of law or fact.

This is a permissive rule — parties may join together but are not required to. A common example is a car accident involving multiple victims: they can all join as co-plaintiffs against the same defendant because their claims arise from the same occurrence (the accident) and share common factual questions (negligence, causation).

Even when joinder is proper under Rule 20, the court retains discretion to sever claims or order separate trials under Rule 42 if joint proceedings would be prejudicial, confusing, or unwieldy. Each party's claims must independently satisfy subject-matter jurisdiction (though supplemental jurisdiction under § 1367 may apply to some parties' claims in certain situations, as clarified by Exxon Mobil v. Allapattah).