Rule 18. Joinder of Claims
(a) In General. A party asserting a claim, counterclaim, crossclaim, or third-party claim may join, as independent or alternative claims, as many claims as it has against an opposing party.
(b) Joinder of Contingent Claims. A party may join two claims even though one of them is contingent on the disposition of the other; but the court may grant relief only in accordance with the parties' relative substantive rights. in particular, a plaintiff may state a claim for money and a claim to set aside a conveyance that is fraudulent as to that plaintiff, without first obtaining a judgment for the money.
(As amended Feb. 28, 1966, eff. July 1, 1966; Mar. 2, 1987, eff. Aug. 1, 1987; Apr. 30, 2007, eff. Dec. 1, 2007.)

Rule 19. Required Joinder of Parties
(a) Persons Required to Be Joined if Feasible.
(1) Required Party. A person who is subject to service of process and whose joinder will not deprive the court of subjectmatter jurisdiction must be joined as a party if:
(A) in that person's absence, the court cannot accord complete relief among existing parties; or
(B) that person claims an interest relating to the subject of the action and is so situated that disposing of the action in the person's absence may:
(i) as a practical matter impair or impede the person's ability to protect the interest; or
(ii) leave an existing party subject to a substantial risk of incurring double, multiple, or otherwise inconsistent obligations because of the interest.
(2) Joinder by Court Order. If a person has not been joined as required, the court must order that the person be made a party. A person who refuses to join as a plaintiff may be made either a defendant or, in a proper case, an involuntary plaintiff.
(3) Venue. If a joined party objects to venue and the joinder would make venue improper, the court must dismiss that party.

(b) When Joinder Is Not Feasible. If a person who is required to be joined if feasible cannot be joined, the court must determine whether, in equity and good conscience, the action should proceed among the existing parties or should be dismissed. The factors for the court to consider include:
(1) the extent to which a judgment rendered in the person's absence might prejudice that person or the existing parties;
(2) the extent to which any prejudice could be lessened or avoided by:
(A) protective provisions in the judgment;
(B) shaping the relief; or
(C) other measures;
(3) whether a judgment rendered in the person's absence would be adequate; and
(4) whether the plaintiff would have an adequate remedy if the action were dismissed for nonjoinder.

(c) Pleading the Reasons for Nonjoinder. When asserting a claim for relief, a party must state:
(1) the name, if known, of any person who is required to be joined if feasible but is not joined; and
(2) the reasons for not joining that person.
(d) Exception for Class Actions. This rule is subject to Rule 23.
(As amended Feb. 28, 1966, eff. July 1, 1966; Mar. 2, 1987, eff. Aug. 1, 1987; Apr. 30, 2007, eff. Dec. 1, 2007.)

Rule 20. Permissive Joinder of Parties
(a) Persons Who May Join or Be Joined.
(1) Plaintiffs. Persons may join in one action as plaintiffs if:
(A) they assert any right to relief jointly, severally, or in the alternative with respect to or arising out of the same transaction, occurrence, or series of transactions or occurrences; and
(B) any question of law or fact common to all plaintiffs will arise in the action.
(2) Defendants. Persons—as well as a vessel, cargo, or other property subject to admiralty process in rem一may be joined in one action as defendants if:
(A) any right to relief is asserted against them jointly, severally, or in the alternative with respect to or arising out of the same transaction, occurrence, or series of transactions or occurrences; and
(B) any question of law or fact common to all defendants will arise in the action.
(3) Extent of Relief. Neither a plaintiff nor a defendant need be interested in obtaining or defending against all the relief demanded. The court may grant judgment to one or more plaintiffs according to their rights, and against one or more defendants according to their liabilities.
(b) Protective Measures. The court may issue orders一includ-ing an order for separate trials—to protect a party against embarrassment, delay, expense, or other prejudice that arises from including a person against whom the party asserts no claim and who asserts no claim against the party.
(As amended Feb. 28, 1966, eff. July 1, 1966; Mar. 2, 1987, eff. Aug. 1, 1987; Apr. 30, 2007, eff. Dec. 1, 2007.)

Rule 21. Misjoinder and Nonjoinder of Parties
Misjoinder of parties is not a ground for dismissing an action. On motion or on its own, the court may at any time, on just terms, add or drop a party. The court may also sever any claim against a party.
(As amended Apr. 30, 2007, eff. Dec. 1, 2007.)

Rule 22. Interpleader
(a) Grounds.
(1) By a Plaintiff. Persons with claims that may expose a plaintiff to double or multiple liability may be joined as defendants and required to interplead. Joinder for interpleader is proper even though:
(A) the claims of the several claimants, or the titles on which their claims depend, lack a common origin or are adverse and independent rather than identical; or
(B) the plaintiff denies liability in whole or in part to any or all of the claimants.
(2) By a Defendant. A defendant exposed to similar liability may seek interpleader through a crossclaim or counterclaim.
(b) Relation to Other Rules and Statutes. This rule supplements一and does not limit—the joinder of parties allowed by Rule 20. The remedy this rule provides is in addition to一and does not supersede or limit—the remedy provided by 28 U.S.C. §§1335, 1397, and 2361. An action under those statutes must be conducted under these rules.
(As amended Dec. 29, 1948, eff. Oct. 20, 1949; Mar. 2, 1987, eff. Aug. 1, 1987; Apr. 30, 2007, eff. Dec. 1, 2007.)

Rule 23. Class Actions
(a) Prerequisites. One or more members of a class may sue or be sued as representative parties on behalf of all members only if:
(1) the class is so numerous that joinder of all members is impracticable;
(2) there are questions of law or fact common to the class;
(3) the claims or defenses of the representative parties are typical of the claims or defenses of the class; and
(4) the representative parties will fairly and adequately protect the interests of the class.

(b) Types of Class Actions. A class action may be maintained if Rule 23(a) is satisfied and if:
(1) prosecuting separate actions by or against individual class members would create a risk of:
(A) inconsistent or varying adjudications with respect to individual class members that would establish incompatible standards of conduct for the party opposing the class; or
(B) adjudications with respect to individual class members that, as a practical matter, would be dispositive of the interests of the other members not parties to the individual adjudications or would substantially impair or impede their ability to protect their interests;
(2) the party opposing the class has acted or refused to act on grounds that apply generally to the class, so that final injunctive relief or corresponding declaratory relief is appropriate respecting the class as a whole; or
(3) the court finds that the questions of law or fact common to class members predominate over any questions affecting only individual members, and that a class action is superior to other available methods for fairly and efficiently adjudicating the controversy. The matters pertinent to these findings include:
(A) the class members' interests in individually controlling the prosecution or defense of separate actions;
(B) the extent and nature of any litigation concerning the controversy already begun by or against class members;
(C) the desirability or undesirability of concentrating the litigation of the claims in the particular forum; and
(D) the likely difficulties in managing a class action.

(c) Certification Order; Notice to Class Members; Judgment; Issues Classes; Subclasses.
(1) Certification Order.
(A) Time to Issue. At an early practicable time after a person sues or is sued as a class representative, the court must determine by order whether to certify the action as a class action.
(B) Defining the Class; Appointing Class Counsel. An order that certifies a class action must define the class and the class claims, issues, or defenses, and must appoint class counsel under Rule 23(g).
(C) Altering or Amending the Order. An order that grants or denies class certification may be altered or amended before final judgment.