For these and many other reasons, parties to litigation will frequently wish to change the positions in their pleadings as the case proceeds. In federal court, their opportunities to do so are governed by Rule 15 of the Federal Rules of Civil Procedure. Rule 15(a) addresses two types of amendments: those filed “as a matter of course” and those allowed by leave of court (or consent of the adverse party).
Rule 15(a) gives all parties the opportunity to amend once as a matter of course, that is, of right, without having to make a motion and have it granted by the judge. The first part of Rule 15(a)(1) provides,
A party may amend its pleading once as a matter of course within:
(A) 21 days after serving it, or
(B) if the pleading is one to which a responsive pleading is required, 21 days after service of a responsive pleading or 21 days after service of a motion under Rule 12(b), (e), or (f), whichever is earlier.
Under this provision, the time to amend differs for complaints and answers. Typically, an answer does not require a responsive pleading, so the time to amend as a matter of course is governed by Rule 15(a)(1)(A). Under that subsection, the defendant will have twenty-one days after serving the answer to amend it without leave of court.
If a pleading does require a responsive pleading, subsection 15(a)(1)(B) provides that the party (typically the plaintiff) may amend within twenty-one days after the answer or (instead) a pre-answer motion under Rule 12 is served on her. Suppose that Mary files a complaint against Potter on March 1, 2012, and Potter serves his answer to the complaint on March 17, 2012. Mary may amend her complaint without leave of court within twenty-one days after that. Or, if Potter files a Rule 12(b)(2) motion to dismiss for lack of personal jurisdiction instead of an answer, Mary would similarly have twenty-one days after service of that motion to amend without leave.
This provision, allowing a plaintiff a short time to amend without leave after receiving the defendant’s answer or motion makes good sense, because the defendant’s pre-answer motion or answer may raise objections or defenses that can be addressed by a quick amendment. Suppose that Mary sues for indirect infliction of emotional distress in a state that recognizes such claims only if the plaintiff was in the zone of physical danger from the defendant’s conduct. If she fails to allege that she was in the zone of danger, the defendant might move to dismiss under Rule 12(b)(6) on that ground. If the plaintiff was in the zone of danger, but had simply failed to allege it, she could fix the problem quickly by filing an amended complaint that includes this allegation. She could also make any other changes she wished to, even if not triggered by the defendant’s motion. She might add more claims, or add some and drop some, change some factual allegations, or file a completely revised complaint. She need not file a motion to amend; the very point of an amendment “as a matter of course” is that you have a right to do it. You don’t have to ask anybody, you just file it.