During oral argument about Montana v. Wyoming this month, the Supreme Court was doing something rare: it was in session as a trial court and its task was to decide how to divvy up water from tributaries to the Yellowstone River.

The Constitution gives the court “original jurisdiction” in cases “affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party,” as in the water case. These cases have a musty aura, but the opposite is true. They are an example of how the court’s understanding of the Constitution is persistently evolving, with few paying any attention.

The case of Marbury v. Madison arose on the original docket. Chief Justice John Marshall declared there that Congress couldn’t enlarge or reduce the court’s original jurisdiction. For 200 years, however, Congress has basically reduced it, giving lower courts authority to hear suits in nearly all of these areas.

The Supreme Court has not complained, but it has retained exclusive authority to resolve what it has called “delicate and grave” disputes between states. Even in those matters, though, the court frequently rejects cases on the fuzzy grounds that they aren’t “appropriate.”