Subject-Matter Jurisdiction
Can a federal court hear this kind of case at all? It's the first question in every Civ Pro fact pattern, it can never be waived, and the court can raise it on its own—even on appeal.
The two doors into federal court
A claim arrives in federal court
It needs exactly one ticket—either door works.
Does the claim arise under federal law?
Federal question, §1331. Citizenship and dollar amount are irrelevant.
Complete diversity—and more than $75,000?
Diversity, §1332. Every plaintiff diverse from every defendant, amount exceeds $75,000.
No SMJ—state court
Unless a related claim can carry it in on supplemental jurisdiction (§1367).
Federal question
The claim must arise under federal law, and the federal issue must appear on the face of a well-pleaded complaint—as part of the plaintiff's own claim (Louisville & Nashville R.R. v. Mottley). An anticipated federal defense—even a certain one—doesn't count.
Lana sues Rosa's café under the federal ADA—arising under, done. But if Lana sues on a state-law lease and merely predicts Rosa will raise federal preemption as a defense, there's no federal question.
Diversity
Complete diversity (Strawbridge v. Curtiss): every plaintiff must be diverse from every defendant, measured at the time of filing. Plus the amount in controversy must exceed $75,000, exclusive of interest and costs.
| Party | Citizenship | Watch for |
|---|---|---|
| Individual | Domicile—physical presence + intent to remain | You keep the old domicile until you truly acquire a new one |
| Corporation | Every state of incorporation AND principal place of business (§1332(c)(1)) | PPB = the “nerve center” (headquarters), not the biggest plant (Hertz v. Friend) |
| Partnership / LLC | Every state of every member or partner | One member sharing a state with the other side kills diversity |
| Class action | Only the named representative counts | Absent members' citizenship is ignored |
Lana (State A) sues Liv (State B) and Rosa (State B): complete diversity. Add Noodle (State A) as a defendant and it's destroyed. And Noodle Corp.—incorporated in Delaware, headquartered in State A—is a citizen of both, so Lana (State A) can't sue it in diversity at all.
Amount in controversy & aggregation
The plaintiff's good-faith allegation controls unless it's a legal certainty the recovery can't exceed $75,000. $75,000 exactly fails—the claim must exceed it.
- One plaintiff stacks all claims against one defendant—related or not ($50k contract + $30k tort = $80k ✓)
- Claims against jointly liable defendants counted once against all
- Two plaintiffs pooling separate claims (Lana's $50k + Noodle's $30k ✗)
- One plaintiff splitting claims across separately liable defendants
Supplemental jurisdiction
Once one claim is properly in, related claims with no ticket of their own ride along if they share a common nucleus of operative fact—the same transaction or occurrence (United Mine Workers v. Gibbs).
§1367(b)—the diversity carve-out. In a case that's in federal court only on diversity, the plaintiff's claims against parties joined under Rules 14, 19, 20, or 24 get no supplemental help. Defendants' claims (counterclaims, crossclaims, impleader claims) are unaffected.
§1367(c)—discretion. The court may decline supplemental jurisdiction—most commonly after dismissing every original-jurisdiction claim early.
Compulsory counterclaims always fit §1367(a)—same transaction by definition. Permissive counterclaims usually need their own ticket. And a plaintiff can never use §1367 to smuggle in a claim that would have failed complete diversity from the start.
Lana's federal-question suit against Rosa carries Lana's related state-law claim—common nucleus, easy. But in a pure diversity case, Lana can't add a $20,000 claim against impleaded Noodle: plaintiff's claim, §1367(b), blocked.
Removal
Only defendants remove; all must join; the case goes to the federal district embracing the state court; and it must have been filable in federal court originally. File the notice within 30 days of service of the first removable paper.
In-state defendant rule (§1441(b)(2)): no diversity removal if any properly joined defendant is a citizen of the forum state.
One-year cap: no diversity removal more than one year after the case began (absent the plaintiff's bad faith).
Plaintiffs never remove—not even to escape a counterclaim.
Wrong court? The fix is remand (§1447(c)): jurisdictional defects can send it back anytime; any other defect must be raised within 30 days of removal.
Lana (State A) sues Rosa (State B) for $100,000 in a State B state court. Rosa can't remove—she's a citizen of the forum. Flip the courthouse to State A and Rosa removes within 30 days.
Where the points are
The traps examiners actually set.
- Most tested
- Complete diversity with a corporation's dual citizenship; the well-pleaded complaint rule (federal defenses don't count); §1367(b) blocking a plaintiff's supplemental claims in diversity cases; the in-state defendant removal bar.
- Classic traps
- $75,000 exactly (fails—must exceed); domicile measured at filing, not at the events; “nerve center” = headquarters, not the biggest factory; two plaintiffs aggregating (never); a plaintiff removing after a counterclaim (plaintiffs never remove); SMJ “waived” by consent (impossible).
Keep going: SMJ flowchart Civ Pro Attack Sequences Joinder deep dive