Pretrial procedures - Joinder of parties and claims (includi...

Learning Outcomes

This article explains joinder of parties and claims in federal civil actions for MBE purposes, including:

• identifying when parties may be permissively joined under Rule 20 and when parties are required under Rule 19, with attention to “necessary” versus “indispensable” status and feasibility of joinder;
• distinguishing joinder of multiple claims under Rule 18 from counterclaims and crossclaims under Rule 13, and recognizing which counterclaims are compulsory on the exam;
• evaluating when third‑party practice (impleader) under Rule 14 is proper, which derivative claims may be asserted among existing and third‑party parties, and how timing and jurisdictional rules affect impleader;
• applying Rule 23’s numerosity, commonality, typicality, and adequacy requirements, the different Rule 23(b) categories, and basic CAFA and amount‑in‑controversy rules to class action fact patterns;
• analyzing how subject matter jurisdiction and supplemental jurisdiction under 28 U.S.C. § 1367 interact with joinder devices in diversity and federal question cases, and spotting when § 1367(b) bars plaintiffs’ claims against parties joined under Rules 14, 19, 20, or 24;
• practicing step‑by‑step exam analysis for joinder questions by first testing the applicable joinder rule, then confirming personal jurisdiction, subject matter jurisdiction, and any supplemental jurisdiction limitations.

MBE Syllabus

For the MBE, you are required to understand joinder mechanisms and their jurisdictional limits, with a focus on the following syllabus points:

• Permissive joinder of parties under Rule 20 and compulsory joinder under Rule 19
• Joinder of multiple claims under Rule 18 and related counterclaims and crossclaims
• Third-party practice (impleader) under Rule 14
• Class actions under Rule 23, including types of class actions and certification requirements
• How joinder interacts with federal subject matter jurisdiction, including diversity, amount in controversy, and supplemental jurisdiction under 28 U.S.C. § 1367
• Restrictions on supplemental jurisdiction for claims by plaintiffs against parties joined under Rules 14, 19, 20, and 24

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Under Rule 20, which is required for permissive joinder of parties?
   1. Claims must arise out of the same transaction or occurrence and share a common question of law or fact.
   2. All parties must be indispensable.
   3. All claims must arise under federal law.
   4. All parties must be citizens of different states.
2. In a federal diversity class action, which is true regarding the amount-in-controversy requirement?
   1. Every class member must meet the amount-in-controversy requirement.
   2. Only the named class representative(s) must meet the amount-in-controversy requirement.
   3. Class actions are not permitted in diversity cases.
   4. All class members must be citizens of the same state.
3. Under Rule 14 (impleader), a defendant may bring in a third-party defendant only if:
   1. The third-party defendant is jointly liable with the defendant to the plaintiff.
   2. The third-party defendant may be liable to the defendant for all or part of the plaintiff’s claim.
   3. The third-party defendant is a necessary party.
   4. The third-party defendant is a class member.
4. In a diversity case, which of the following joined claims is most clearly barred by the supplemental jurisdiction statute?
   1. A compulsory counterclaim by the defendant against the plaintiff.
   2. A claim by the original plaintiff against a third-party defendant joined under Rule 14.
   3. A claim by a third-party defendant against the original defendant.
   4. A crossclaim between co-defendants arising from the same accident.
5. Which of the following is not required to certify a Rule 23(b)(3) damages class action?
   1. Numerosity
   2. Commonality
   3. Typicality
   4. Proof that each class member’s damages exceed $75,000

Introduction

Joinder rules determine how many people and how many disputes can be handled in a single federal civil action. Used correctly, joinder promotes efficiency and consistent outcomes; used incorrectly, it can destroy subject matter jurisdiction or lead to dismissal for failure to join required parties.

The Federal Rules of Civil Procedure separate:

• How parties can be added (Rules 19, 20, 14, 24)
• How claims can be added (Rule 18 and Rule 13 counterclaims/crossclaims)
• How representative suits proceed (Rule 23 class actions)

All of these operate against a jurisdictional backdrop: every claim in federal court must be supported by either original subject matter jurisdiction (diversity or federal question) or supplemental jurisdiction, and the supplemental jurisdiction statute (§ 1367) imposes special limits in diversity-only cases.

Key Term: Permissive Joinder
Permissive joinder allows multiple plaintiffs or defendants to appear in one action when their claims arise out of the same transaction or occurrence (or series of transactions or occurrences) and share at least one common question of law or fact, as provided by Rule 20.

Key Term: Compulsory Joinder
Compulsory joinder, governed by Rule 19, identifies parties who must be joined if feasible because their absence would prevent complete relief or risk prejudice to them or existing parties. If such a person cannot be joined, the court decides whether to dismiss or proceed without them.

Key Term: Necessary Party
A necessary (or “required”) party under Rule 19(a) is a person who should be joined if feasible because complete relief cannot be accorded without them, or because proceeding without them may impair their interests or expose existing parties to multiple or inconsistent obligations.

Key Term: Indispensable Party
An indispensable party is a necessary party whose absence is so prejudicial under Rule 19(b) that the court must dismiss rather than proceed; the action cannot, “in equity and good conscience,” continue without that person.

Key Term: Joinder of Claims
Joinder of claims (Rule 18) allows a party asserting any claim, counterclaim, crossclaim, or third‑party claim to join as many additional claims as it has against an opposing party, related or unrelated, subject to subject matter jurisdiction limits.

Key Term: Counterclaim
A counterclaim is a claim asserted by a defending party against an opposing party. Some counterclaims are compulsory (arising out of the same transaction or occurrence) and must be raised or are waived; others are permissive.

Key Term: Crossclaim
A crossclaim is a claim by one party against a co‑party (for example, one defendant against another) that arises out of the same transaction or occurrence as the original action or a counterclaim.

Key Term: Impleader (Third-Party Practice)
Impleader under Rule 14 allows a defending party to bring in a third-party defendant who may be liable to the defending party for all or part of the plaintiff’s claim, typically on a derivative theory such as indemnity or contribution.

Key Term: Third-Party Defendant (TPD)
A third‑party defendant is a new party brought into the case by a defending party under Rule 14, against whom the defending party asserts derivative liability for the plaintiff’s claim.

Key Term: Class Action
A class action is a lawsuit in which one or more named plaintiffs (class representatives) sue on behalf of a larger group with common legal or factual issues, subject to Rule 23’s requirements.

Key Term: Class Representative
A class representative is a named plaintiff (or defendant) whose claim or defense is typical of the class and who will fairly and adequately protect class interests.

Key Term: Supplemental Jurisdiction
Supplemental jurisdiction under 28 U.S.C. § 1367 allows a federal court with original jurisdiction over at least one claim to hear additional related claims forming part of the same case or controversy, subject to limitations in diversity-only cases.

With these core concepts in place, the rest of the article examines each major joinder mechanism and then ties them back to subject matter jurisdiction and common MBE patterns.

Permissive Joinder of Parties (Rule 20)

Permissive joinder is about efficiency: when disputes are sufficiently related, they can be litigated together.

Rule 20 permits multiple plaintiffs or defendants in one action if:

• The claims arise out of the same transaction, occurrence, or series of transactions or occurrences; and
• There is at least one common question of law or fact.

It applies separately to:

• Joinder of plaintiffs (multiple plaintiffs suing one or more defendants)
• Joinder of defendants (one or more plaintiffs suing multiple defendants)

Each joined party must assert (or be subject to) at least one claim satisfying these two elements. Other claims between the same parties can then be joined under Rule 18 if jurisdictionally proper.

Examples where joinder is typically proper include:

• Multiple persons injured in the same auto accident
• Multiple purchasers suing over the same defective product model
• Multiple employees challenging the same discriminatory policy

Joinder under Rule 20 is permissive, not mandatory. A plaintiff may choose to file separate suits even when Rule 20 would allow joinder, though the court can consolidate related cases.

Jurisdictional Overlay

Even if Rule 20 permits joinder, the court must have:

• Personal jurisdiction over each joined defendant; and
• Subject matter jurisdiction over each claim, either directly (diversity or federal question) or via supplemental jurisdiction.

In diversity cases, the supplemental jurisdiction statute is generous on amount in controversy but strict on complete diversity:

• If one plaintiff’s claim exceeds $75,000 and complete diversity exists, a second plaintiff with a related claim **below**$75,000 can usually piggy‑back using supplemental jurisdiction.
• But supplemental jurisdiction cannot be used to cure a lack of complete diversity created by permissive joinder under Rule 20.

Exam Tip:
In a diversity case, if adding a plaintiff or defendant under Rule 20 would make any plaintiff a citizen of the same state as any defendant, supplemental jurisdiction cannot save that joinder. The case must either drop that party or be dismissed.

Misjoinder

If parties are “misjoined” (Rule 20 not satisfied), Rule 21 allows the court to:

• Drop misjoined parties; or
• Sever claims into separate actions.

Misjoinder is not a ground for dismissing the entire action.

Compulsory Joinder of Parties (Rule 19)

Key Term: Compulsory Joinder
The required joining of certain parties to an action when their absence would prevent complete relief or risk serious prejudice, governed by Rule 19.

Rule 19 identifies parties who are so connected to the controversy that the case usually should not proceed without them.

The analysis proceeds in three steps:

1. Is the absent person “necessary” (required) under Rule 19(a)?
2. If necessary, is joinder feasible?
3. If necessary but joinder is not feasible, is the party “indispensable” such that the case must be dismissed?

Step 1: Necessary (Required) Parties – Rule 19(a)

A party is necessary if any of the following is true:

• In that person’s absence, the court cannot accord complete relief among existing parties (for example, co‑owners both need to be bound to quiet title).
• The person claims an interest relating to the subject of the action and proceeding without them may:
  • Impair or impede their ability to protect that interest; or
  • Leave an existing party subject to a substantial risk of multiple or inconsistent obligations.

Classic necessary parties include:

• Joint obligees or joint promisees on the same contract
• Co‑owners of real property in an action affecting title or possession
• Persons claiming rights in the same limited fund

Joint tortfeasors are generally not necessary parties: a plaintiff can typically sue one tortfeasor without joining all others.

Step 2: Feasibility of Joinder

The court then asks whether the necessary party can be joined. Joinder is feasible only if:

• The court has personal jurisdiction over the party; and
• Joinder will not destroy subject matter jurisdiction (for example, complete diversity).

If joinder is feasible, the court must order that the person be joined.

If joinder is not feasible (for example, adding them would destroy diversity or they are immune from suit), the court moves to Rule 19(b).

Step 3: Indispensable Parties – Rule 19(b)

Under Rule 19(b), the court decides whether, “in equity and good conscience,” the action should:

• Proceed among the existing parties; or
• Be dismissed because the absent party is indispensable.

Relevant factors include:

• The extent of prejudice to the absent person or existing parties
• Whether prejudice can be lessened by shaping relief (for example, issuing a declaratory judgment rather than damages)
• Whether a judgment in the person’s absence would be adequate
• Whether the plaintiff would have an adequate remedy if the action is dismissed

If dismissal is required, the absent necessary party is labeled indispensable, and the action cannot proceed.

Exam Warning

Watch the terminology: “necessary” = required if feasible under Rule 19(a); “indispensable” = a necessary party whose absence forces dismissal under Rule 19(b).

Joinder of Claims (Rule 18) and Related Devices

Key Term: Joinder of Claims
Rule 18 allows a party to join any additional claims it has against an opposing party once there is at least one properly asserted claim between them, regardless of whether the claims are related—subject to subject matter jurisdiction.

Once a party has made any claim against an opposing party (complaint, counterclaim, crossclaim, or third‑party complaint), that party may join as many other claims as it has against that opponent. The Rules do not require logical relatedness between claims under Rule 18.

However, every claim still needs:

• A basis for original subject matter jurisdiction; or
• Supplemental jurisdiction (same case or controversy).

Counterclaims and Crossclaims (Rule 13) – Brief Overview

Though Rule 13 is not itself a joinder rule, the MBE frequently tests it alongside Rule 18:

• Compulsory counterclaim: A claim by a defendant against a plaintiff that arises out of the same transaction or occurrence as the plaintiff’s claim. It must be asserted or it is waived (unless it was not yet mature). Supplemental jurisdiction usually covers it.
• Permissive counterclaim: Any other counterclaim. It requires its own basis for subject matter jurisdiction (no automatic supplemental jurisdiction).
• Crossclaim: A claim by one party against a co‑party (for example, D1 v. D2) that arises out of the same transaction or occurrence as the original action or a counterclaim. Crossclaims are always permissive, never compulsory.

Key Term: Counterclaim
A claim by a defending party against an opposing party; it may be compulsory (same transaction or occurrence) or permissive (unrelated).

Key Term: Crossclaim
A claim by one party against a co‑party arising out of the same transaction or occurrence as the original claim or counterclaim.

Rule 18 then allows additional claims to be joined on top of these.

Third-Party Practice (Impleader) (Rule 14)

Key Term: Impleader (Third-Party Practice)
A procedure under Rule 14 allowing a defending party to bring in a third party who may be liable to the defending party for all or part of the plaintiff’s claim, usually based on indemnity, contribution, or similar derivative liability.

Key Term: Third-Party Defendant (TPD)
The newly joined party brought into the lawsuit by a defending party under Rule 14 to answer derivative claims relating to the plaintiff’s demand.

A defending party (often the original defendant) may implead a third‑party defendant if:

• The TPD may be liable to the defending party for all or part of the plaintiff’s claim; and
• The asserted liability is derivative of the defending party’s liability to the plaintiff (for example, indemnity, contribution, contractual duty to reimburse).

Impleader is not a device for bringing in someone who is alleged to be directly liable to the plaintiff instead of the defendant.

• A defendant can implead a supplier who must indemnify it for defective goods.
• A driver sued for negligence can implead a co‑tortfeasor for contribution (if state law allows contribution).
• But a defendant cannot implead a different driver simply on the theory that the other driver, not the defendant, was the one who injured the plaintiff. That is a separate potential defendant for the plaintiff, not someone derivatively liable to the defendant.

Any defending party may use Rule 14:

• Original defendants
• Third‑party defendants (who may implead a fourth party)
• Plaintiffs, when defending against a counterclaim

What Claims Can Be Asserted

Once a TPD is joined:

• The third‑party plaintiff asserts the derivative claim against the TPD (the impleader claim).
• The TPD may assert defenses against the third‑party plaintiff and the original plaintiff.
• The TPD may assert claims against the original plaintiff arising out of the same transaction or occurrence.
• The original plaintiff may, in turn, assert claims directly against the TPD arising out of the same transaction or occurrence (subject to subject matter jurisdiction limitations, especially in diversity cases).

Exam Warning

Impleader is not proper if the third party is only alleged to be directly liable to the plaintiff, not to the defendant. That scenario calls for the plaintiff to sue the additional defendant directly, usually under Rule 20 permissive joinder.

Timing

A defending party may file a third‑party complaint:

• Without leave of court if filed within 14 days after serving its answer;
• Otherwise, only with the court’s leave.

Subject Matter and Supplemental Jurisdiction

Impleader often arises in diversity cases where the third‑party plaintiff and third‑party defendant are citizens of the same state:

• The impleader claim itself is generally within the court’s supplemental jurisdiction if it arises from the same case or controversy as the original claim.
• The restrictions in § 1367(b) apply only to claims by plaintiffs in diversity-only cases. A defendant’s impleader claim against a non‑diverse TPD is usually permitted.

However, in diversity cases:

• Claims by the original plaintiff against the TPD cannot rely on supplemental jurisdiction if that would destroy complete diversity. Those claims need an independent basis of jurisdiction (diversity or federal question).

Class Actions (Rule 23)

Key Term: Class Action
A lawsuit in which one or more plaintiffs sue (or are sued) on behalf of a larger group, with the judgment binding absent class members, if Rule 23’s requirements are met.

Key Term: Class Representative
The named party in a class action whose claim is typical of the class and who adequately protects the class’s interests.

Class actions allow adjudication of many similar small claims in a single proceeding and prevent inconsistent results.

Rule 23(a) Prerequisites – “Numerosity, Commonality, Typicality, Adequacy”

Every class action must satisfy all four Rule 23(a) requirements:

Key Term: Numerosity
Numerosity requires that the class be so numerous that joinder of all members is impracticable. There is no fixed number, but classes with dozens or hundreds of members usually qualify.

Key Term: Commonality
Commonality requires at least one common question of law or fact whose answer will resolve an issue central to all class members’ claims in one stroke.

Key Term: Typicality
Typicality requires that the class representatives’ claims or defenses be typical of those of the class—arising from the same event, practice, or course of conduct and based on the same legal theory.

Key Term: Adequacy
Adequacy requires that the class representatives and class counsel fairly and adequately protect the interests of the class, without conflicts of interest and with sufficient competence.

Commonality is often the hardest element after the Supreme Court’s decision in Wal‑Mart v. Dukes. The common question must be capable of classwide resolution; it is not enough that class members raise similar legal theories if the factual issues require individualized determinations.

Rule 23(b) Categories

In addition, the case must fit into one of three Rule 23(b) categories:

• Rule 23(b)(1) – Risk of inconsistent adjudications or impairment of others’ interests
  • For example, separate suits regarding rights in a limited fund.
• Rule 23(b)(2) – Defendant has acted on grounds generally applicable to the class, making final injunctive or declaratory relief appropriate
  • Common in civil rights or institutional reform cases. Not suited for individualized money damages.
• Rule 23(b)(3) – Common questions predominate over individual ones and a class action is superior to other methods
  • This is the usual vehicle for damages class actions (consumer fraud, securities fraud, etc.).

Only Rule 23(b)(3) classes require:

• Notice to class members; and
• An opportunity to “opt out.”

Jurisdictional Issues in Class Actions

For class actions in federal court:

• Federal question class actions: Subject matter jurisdiction is straightforward if the class asserts federal claims.
• Traditional diversity class actions:
  • Only the citizenship of the named class representatives counts for diversity.
  • Under current Supreme Court case law, as long as at least one named representative’s claim exceeds $75,000, other class members’ diverse claims can come in under supplemental jurisdiction, even if their individual claims are smaller.

Exam Fact:
The citizenship of unnamed class members is disregarded for diversity, and only the named representatives’ claims are considered for the amount‑in‑controversy requirement.

There is also a special jurisdictional statute (often called CAFA) that gives federal courts jurisdiction over large class actions with:

• Minimal diversity (any class member diverse from any defendant); and
• An aggregated amount in controversy over $5 million; and
• At least 100 class members.

The MBE sometimes tests CAFA at a surface level.

Supplemental Jurisdiction and Joinder

Key Term: Supplemental Jurisdiction
Supplemental jurisdiction allows federal courts to hear additional claims that form part of the same Article III case or controversy as a claim within original jurisdiction, subject to statutory limitations in diversity-only cases.

Supplemental jurisdiction under § 1367 works in two main steps:

1. Under § 1367(a), federal courts may hear additional claims that are so related to the original claim that they form part of the same case or controversy (often, same transaction or occurrence).
2. Under § 1367(b), in diversity-only cases, there are important restrictions on using supplemental jurisdiction for certain claims by plaintiffs.

Where Supplemental Jurisdiction Freely Applies

Supplemental jurisdiction is usually available for:

• Compulsory counterclaims (same transaction or occurrence)
• Crossclaims between co‑parties (same transaction or occurrence)
• Impleader claims by defendants against third‑party defendants (same case or controversy)
• Claims by third‑party defendants back against the third‑party plaintiffs arising from the same transaction

In federal question cases, § 1367(b) does not apply, so supplemental jurisdiction is broad.

§ 1367(b) – Restrictions in Diversity-Only Cases

When original jurisdiction is based solely on diversity, supplemental jurisdiction may not be used for:

• Claims by plaintiffs against persons made parties under Rules 14 (impleader), 19 (compulsory joinder), 20 (permissive joinder), or 24 (intervention);
• Claims by persons proposed to be joined as plaintiffs under Rule 19;
• Claims by persons seeking to intervene as plaintiffs under Rule 24,

if exercising supplemental jurisdiction would be inconsistent with diversity requirements.

Practical implications for the MBE:

• A diversity plaintiff cannot use supplemental jurisdiction to sue a third‑party defendant (Rule 14) if that would defeat complete diversity.
• A diversity plaintiff cannot use supplemental jurisdiction to join additional defendants under Rule 20 if that would defeat complete diversity.
• But supplemental jurisdiction can be used by defendants to implead non‑diverse third‑party defendants, or to assert crossclaims, so long as the claims are related.

Multiple Plaintiffs and Amount in Controversy

In diversity cases with multiple plaintiffs:

• If one plaintiff’s claim independently meets the $75,000 threshold and complete diversity exists, other plaintiffs with related claims below$75,000 can generally be joined under supplemental jurisdiction (assuming they do not violate § 1367(b)).

Worked Example 1.1

Scenario:
Plaintiff A and Plaintiff B are injured in the same car accident involving Defendant X. Both wish to sue X in federal court for negligence.

Question:
Can A and B join as plaintiffs in a single action against X?

Answer:
Yes. Under Rule 20, A and B may permissively join as plaintiffs because their claims arise out of the same occurrence (the accident) and share common questions of law or fact (negligence and causation). Subject matter jurisdiction must still be satisfied, but if A’s claim exceeds $75,000 and diversity exists, supplemental jurisdiction can usually cover B’s smaller related claim.

Worked Example 1.2

Scenario:
Defendant Y is sued by Plaintiff C for breach of contract. Y claims that if Y is liable, it is because Supplier Z provided defective goods to Y.

Question:
Can Y bring Z into the action as a third-party defendant?

Answer:
Yes. Y may implead Z under Rule 14, alleging that Z is liable to Y for all or part of C’s claim (for example, contractual indemnity or contribution for defective goods). Z’s liability is derivative of Y’s liability to C, which is exactly what impleader is designed for.

Worked Example 1.3

Scenario:
A group of 1,000 consumers sues a manufacturer for a defective product in a federal diversity action. Only the named plaintiff’s claim exceeds $75,000.

Question:
Is the class action proper in federal court?

Answer:
Yes. For diversity class actions under the traditional rule, only the named class representative(s) must meet the amount‑in‑controversy requirement, and only their citizenship counts for diversity. The citizenship and individual claim amounts of unnamed class members are disregarded for § 1332 purposes, though CAFA may provide an alternative jurisdictional route.

Worked Example 1.4

Scenario:
P sues D in federal diversity court to quiet title to Blackacre. O, a co‑owner of Blackacre, is not joined. O is a citizen of the same state as D.

Question:
Is O a necessary party, and what happens if joining O would destroy diversity?

Answer:
Yes. As a co‑owner of property whose title is at stake, O is a necessary party under Rule 19(a) because complete relief cannot be accorded and O’s interest would be directly affected. If joining O would destroy complete diversity, joinder is not feasible. The court must then apply Rule 19(b) to decide whether O is indispensable. In most quiet title actions, co‑owners are indispensable, so the case will likely be dismissed.

Worked Example 1.5

Scenario:
P, a citizen of State A, sues D, a citizen of State B, in federal diversity court for $200,000 in negligence. D impleads X, a citizen of State A, under Rule 14 for contribution. P then asserts a$50,000 negligence claim directly against X arising from the same accident.

Question:
Which claims are within the court’s subject matter jurisdiction?

Answer:
D’s impleader claim against X is within the court’s supplemental jurisdiction because it arises from the same case or controversy and § 1367(b) does not restrict claims by defendants. However, P’s claim against X is a claim by a plaintiff against a party joined under Rule 14. In a diversity‑only case, § 1367(b) bars supplemental jurisdiction for that claim, and P and X are not diverse. Therefore, P’s claim against X is not within the court’s jurisdiction and must be dismissed or filed in state court.

Worked Example 1.6

Scenario:
Two unrelated passengers on a small plane sue the pilot in federal district court for injuries from a crash. Passenger 1 (P1) claims $95,000; Passenger 2 (P2) claims$5,000. P1 and P2 are citizens of the same state; the pilot is a citizen of another state.

Question:
Should the court dismiss P2’s claim for lack of subject matter jurisdiction?

Answer:
No. The court has diversity jurisdiction over P1’s claim (complete diversity and amount over $75,000). P2’s claim arises out of the same accident and forms part of the same case or controversy, so supplemental jurisdiction can cover it even though it is only for$5,000. The fact that P1 and P2 are co‑citizens does not matter; complete diversity looks across the “v.” between plaintiffs and defendants.

Revision Tip

Always analyze joinder questions in two layers:

• First, does a joinder rule (Rule 19, 20, 14, 23, etc.) permit or require adding the person or claim?
• Second, does the court have subject matter jurisdiction and personal jurisdiction over the joined claims and parties, considering the limits of supplemental jurisdiction in diversity cases?

Key Point Checklist

This article has covered the following key knowledge points:

• Permissive joinder (Rule 20) allows parties to join if their claims arise from the same transaction or occurrence (or series) and share at least one common question of law or fact.
• Compulsory joinder (Rule 19) requires “necessary” parties to be joined if feasible; if joinder is impossible, the court decides whether the party is indispensable and whether the case must be dismissed.
• Rule 18 lets a party join any number of additional claims against an opposing party, but each claim must have a basis for subject matter jurisdiction, either original or supplemental.
• Counterclaims and crossclaims (Rule 13) often work with Rule 18: compulsory counterclaims and crossclaims based on the same transaction or occurrence typically fall within supplemental jurisdiction.
• Impleader (Rule 14) allows a defending party to bring in a third‑party defendant who may be derivatively liable to the defending party for all or part of the plaintiff’s claim; impleader is not a way to add a separate direct defendant for the plaintiff.
• Class actions under Rule 23 require numerosity, commonality, typicality, and adequacy, plus fitting into one of the Rule 23(b) categories. Rule 23(b)(3) damages classes require that common questions predominate and that the class action be superior.
• In diversity class actions, only the citizenship and claim amount of named class representatives matter for § 1332; unnamed class members’ citizenship and claim amounts are generally ignored for traditional diversity.
• Supplemental jurisdiction under § 1367 permits related claims to be heard with an anchor federal claim, but § 1367(b) restricts certain claims by plaintiffs in diversity‑only cases, especially claims against parties joined under Rules 14, 19, 20, or 24.
• In diversity cases, defendants can usually use supplemental jurisdiction to implead non‑diverse third‑party defendants or assert crossclaims, but plaintiffs cannot use supplemental jurisdiction to sue those same parties if that would defeat diversity.
• Misjoinder of parties or claims is not itself a ground for dismissing the whole case; the court can drop or sever misjoined parties under Rule 21.

Key Terms and Concepts

• Permissive Joinder
• Compulsory Joinder
• Necessary Party
• Indispensable Party
• Joinder of Claims
• Counterclaim
• Crossclaim
• Impleader (Third-Party Practice)
• Third-Party Defendant (TPD)
• Class Action
• Class Representative
• Supplemental Jurisdiction
• Numerosity
• Commonality
• Typicality
• Adequacy