Which rule governs permissive joinder of parties in federal courts?

The rule that governs permissive joinder of parties in federal courts is indeed Federal Rule of Civil Procedure 20. This rule allows multiple parties to join together in one action, either as plaintiffs or defendants, if their claims arise out of the same transaction or occurrence and if there are questions of law or fact common to all the parties.

This provision is essential as it facilitates the efficient resolution of disputes where related claims are involved, minimizing the need for multiple lawsuits and conserving judicial resources. It balances the need for judicial efficiency with the interests of individual parties, granting the court discretion to allow or reject such joinder based on the circumstances of each case.

The other options focus on different aspects of federal civil procedure. For instance, Rule 10 addresses the form of pleadings, Rule 15 deals with amendments to pleadings, and Rule 25 focuses on the substitution of parties. Thus, these rules do not pertain to the permissive joinder of parties as defined under Rule 20.