Joinder
Who can be in the lawsuit, and which claims ride along. The examiners love joinder because every device has its own little test—and because supplemental jurisdiction hides underneath all of them.
| Device | Rule | The test in one line |
|---|---|---|
| Claim joinder | FRCP 18 | Join anything against an existing opponent—no relatedness needed |
| Permissive parties | FRCP 20 | Same transaction/occurrence + a common question |
| Required parties | FRCP 19 | Can the case proceed fairly without them? |
| Counterclaims | FRCP 13(a)–(b) | Same T/O = compulsory; anything else = permissive |
| Crossclaims | FRCP 13(g) | Against a co-party, same T/O—never compulsory |
| Impleader | FRCP 14 | Third party owes the defendant indemnity or contribution |
| Intervention | FRCP 24 | Outsider's interest is at stake and unprotected |
| Class actions | FRCP 23 | CANT + a 23(b) category |
Joining claims
Once a party properly asserts one claim against an opponent, they may join every other claim they have against that opponent—related or not. The catch is never the rule; it's jurisdiction: each added claim still needs its own SMJ hook.
Lana properly sues Rosa in federal court over their salsa-distribution contract (diversity). Rule 18 lets Lana tack on her unrelated slander claim—but that claim needs its own jurisdictional basis. Same parties, still diverse, over $75,000? Fine.
Permissive party joinder
Plaintiffs may sue together—or join multiple defendants—when the claims (1) arise out of the same transaction or occurrence (or series) and (2) share a common question of law or fact. Both prongs.
Liv's brakes fail and she plows into Lana's parked car and Rosa's café patio. Lana and Rosa can sue Liv together: one occurrence, common questions. But Noodle, who slipped on Rosa's wet floor last month, can't hop in—different occurrence entirely.
Required parties
A party is required if complete relief is impossible without them, their absence would impair their interest, or it would expose an existing party to double or inconsistent obligations. If joinder is feasible, the court must order it. If it isn't (no PJ, or it would destroy diversity), the court decides in equity and good conscience whether to proceed or dismiss—dismissal means the absentee was indispensable.
Joint tortfeasors are never required parties. Lana can sue Liv alone and leave Rosa out, even if both caused the crash. Joint obligees/obligors and co-owners of property are the fact patterns that DO qualify.
Lana sues Rosa to force the sale of a food truck Rosa co-owns with Noodle. Noodle is required—the truck can't be sold without affecting his interest. If joining him kills diversity, the court weighs pressing on against dismissing.
Counterclaims
- Same transaction or occurrence as the plaintiff's claim
- Use it or lose it—omit it and it's gone forever
- Supplemental jurisdiction automatic (same case or controversy by definition)
- Anything else—unrelated claims welcome
- May be saved for a separate lawsuit
- Needs its own jurisdictional basis
Lana sues Liv over the intersection crash. Liv's claim that Lana dented HER bumper in the same collision is compulsory—now or never. Liv's claim on an unpaid 2019 loan is permissive: bring it (with its own jurisdiction) or save it.
Crossclaims
A claim against a co-party must arise from the same transaction or occurrence as the original action. Crossclaims are never compulsory—you may always save them for later.
Lana sues Liv and Rosa after the delivery-van pileup. Liv may crossclaim against Rosa (“your van hit me first”)—same pileup—but she doesn't have to.
Impleader
A defending party may bring in a nonparty who is or may be liable to the defendant for all or part of the judgment—indemnity or contribution. Serve it as of right within 14 days of the answer; after that, with leave.
Can you implead them?
Would the third party owe the defendant reimbursement if the defendant loses?
Implead. Indemnity (“you owe me all of it”) or contribution (“your share”)—the derivative hook is satisfied.
Is the theory just “it was actually their fault, not mine”?
Not impleader. That's a defense—nothing flows to the defendant. Argue it in the answer.
The §1367(b) trap rides along: in diversity-only cases, the plaintiff's claims against the impleaded party get no supplemental jurisdiction—they need their own ticket. The defendant's impleader claim itself is fine.
Lana sues Rosa after Rosa's espresso machine scalds her. Rosa may implead Noodle Corp., the manufacturer—“if I owe Lana, Noodle Corp. owes me.” Rosa may NOT implead Liv just to say “Liv did it.”
Intervention
Of right: the outsider claims an interest the case would impair, and no existing party adequately represents it. Permissive: a claim or defense sharing a common question—the court's call. Both must be timely.
Lana sues to void Rosa's liquor license. Noodle, Rosa's business partner whose livelihood rides on that license, may intervene of right if Rosa won't adequately protect him.
Class actions
Four 23(a) prerequisites—CANT—plus one 23(b) category. Certification is appealable within 14 days; settlement of a certified class needs court approval.
| Element | What it demands |
|---|---|
| Commonality | Questions of law or fact common to the class |
| Adequacy | Representatives will fairly and adequately protect absent members |
| Numerosity | So many members that ordinary joinder is impracticable |
| Typicality | The representatives' claims are typical of the class's |
| + 23(b)(3) | Common questions predominate and a class action is superior → triggers notice + opt-out rights |
Jurisdiction: only the named representative must be diverse from the defendants, and one representative's claim over $75,000 suffices (absent members ride on supplemental jurisdiction). CAFA separately opens federal court to $5M+ classes on minimal diversity.
Rosa's mislabeled “decaf” sends thousands of customers, Lana included, into sleepless weeks. A (b)(3) class works: whether it was mislabeled predominates over how badly each person slept. Lana, as named plaintiff, must be diverse from Rosa; absent members' citizenship doesn't matter.
Where the points are
The traps examiners actually set.
- Most tested
- Compulsory vs. permissive counterclaims; impleader's derivative-liability requirement; §1367(b) blocking a plaintiff's supplemental claims against impleaded or Rule 20 defendants in diversity cases.
- Classic traps
- “Implead the person who really did it” (must owe the defendant); treating crossclaims as compulsory (never); calling a joint tortfeasor a required party (never); forgetting each Rule 18 claim needs its own jurisdiction; letting an absent class member's citizenship destroy diversity (it doesn't).
Keep going: Life of a Lawsuit quick chat Civ Pro Attack Sequences SMJ deep dive SMJ flowchart