Rule 15.Amended and Supplemental Pleadings.

Rule 15 gives a party one amendment as a matter of course any time before a responsive pleading is served, or within 20 days if no response is required and the case isn't yet on the trial calendar. After that, an amendment needs either the court's leave or the other side's written consent, and leave is to be freely given when justice requires it; a proposed amendment must be submitted in Ramseyer format, with new text underscored and deleted text bracketed and struck through, and must reproduce the entire pleading rather than incorporating the old one by reference. A party has the longer of the original response time or 10 days after service of the amended pleading to respond. During and after trial, issues tried by the parties' consent are treated as if they had been properly pleaded, and the court may allow the pleadings to be amended over an evidentiary objection when doing so serves the merits without prejudicing the objecting party.

The rule also decides when an amendment relates back to the date of the original pleading: either because the statute of limitations itself allows it, because the amendment arises from the same conduct or transaction already pleaded, or because it corrects a mistake about which party was sued and that party had adequate notice and should have known the suit would have named it but for the mistake. Finally, Rule 15 lets a party file a supplemental pleading, with the court's permission, to add events that happened after the original pleading was filed, even if that original pleading had its own defects.