Section. 2.
The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority;—to all Cases affecting Ambassadors, other public Ministers and Consuls;—to all Cases of admiralty and maritime Jurisdiction;—to Controversies to which the United States shall be a Party;—to Controversies between two or more States;— between a State and Citizens of another State,—between Citizens of different States,—between Citizens of the same State claiming Lands under Grants of different States, and between a State, or the Citizens thereof, and foreign States, Citizens or Subjects.
In all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party, the supreme Court shall have original Jurisdiction. In all the other Cases before mentioned, the supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make.
The Trial of all Crimes, except in Cases of Impeachment, shall be by Jury; and such Trial shall be held in the State where the said Crimes shall have been committed; but when not committed within any State, the Trial shall be at such Place or Places as the Congress may by Law have directed.
I am not your lawyer. This is not legal advice.
So, the other way the framers put some guardrails on the courts was to define jurisdiction, but again give Congress the reigns on the details. Courts have jurisdiction only in certain cases. Federal courts can hear constitutional cases (or cases about federal laws), cases about ambassadors, maritime cases, cases in which the United States—a jural entity—is a party, cases between states or people and foreign states or between people from diverse states, basically.
However, the Supreme Court only gets to be a trial court when ambassadors or states are suing. Otherwise, if Congress wants the law enforced or interpreted, it needs to create trial courts.
Further, the federal government needs to create trial courts in any state where it wants federal criminal enforcement, because, says the Constitution, if you rob the bank in Maine, they’ve gotta try you in Maine (except of course there are some workarounds, so don’t get cute. Also, I can’t stress this enough, don’t rob banks and this isn’t legal advice).
Next week, believe it or not, we will get to the last—and therefore most important, right?—section of Article III.