Justice David Souter wrote the Supreme Court opinion that cut the punitive damages in the Exxon Valdez spill to $500 million from $2.5 billion, and it is a surprisingly good read.

Supreme Court opinions often tend toward abstractions, but Souter begins by reminding us of the harrowing facts of March 24, 1989, when a 900-foot supertanker used by Exxon to carry crude oil from the end of the Trans-Alaska Pipeline to the lower 48 states grounded on Bligh Reef, spilling millions of gallons of crude oil into Prince William Sound:

"Its captain was one Joseph Hazelwood, who had completed a 28-day alcohol treatment program while employed by Exxon, as his superiors knew, but dropped out of a prescribed follow-up program and stopped going to Alcoholics Anonymous meetings."

Souter continues, reviewing the evidence presented at the trial: "Witnesses testified that before the Valdez left port on the night of the disaster, Hazelwood downed at least five double vodkas in the waterfront bars, an intake of about 15 ounces of 80-proof alcohol..."

Yet the most important language in the decision may be found in a single footnote.

Andrew Frey, one of the nation's top Supreme Court advocates, who filed a brief in the case on behalf of the American Petroleum Institute, points to footnote 28 as a sign that the court will apply a one-to-one ratio when it comes to punitive damages in virtually all cases.

Footnote 28 cites a 2002 Supreme Court decision involving State Farm—a case argued and won by Frey himself, and the last major previous pronouncement from the court on punitive damages. State Farm said that a one-to-one ratio is appropriate in "all but the most exceptional cases," Souter observed.

So if Exxon Valdez is not exceptional, then what is?

In the footnote, Souter notes that the recovery of $500 million by the Exxon Valdez class member was "substantial. In this case, then, the constitutional outer limit may well by 1:1."

Frey contends: "What they have done here is they have tried to adopt a general rule of thumb."

Indeed, Souter's opinion gives a lengthy treatise on the history of punitive damages, a much broader examination of the subject than the maritime law question presented in the case. He notes that the modern doctrine of punitive damages dates back to at least 1763, and continues from there, in a history that also contrasts American law to practice in Canada and Australia.

Souter notes: "American punitive damages have been the target of audible criticism in recent decades," and observes: "The real problem, it seems, is the stark unpredictability of punitive awards."

And so now, it seems, Corporate America has a number it can rely on: It is one-to-one.

J.B. Howard, the deputy attorney general of the state of Maryland, who wrote a brief on behalf of 34 states, is not happy about that. "We think the court failed to appreciate the egregiousness of Exxon's conduct," he said.

For Howard, and for many people watching the case, the justices failed to make a ruling on the main point in the case. They split four-to-four on whether, under maritime law, the agent of a captain at sea could be held accountable for his actions, if the agent did not countenance those actions.

At issue was an 1818 ruling in a case called the Amiable Nancy, which involved a ship captain who took to pirating while out at sea.

Justice Samuel Alito, who owns a large stake in ExxonMobil stock, did not participate in the case. (He sold part of those holdings earlier this month, according to financial disclosures.)

The even split means that a previous decision from a federal appeals court found that Exxon should be liable for Hazelwood's conduct, even though it did not officially send him out to sea while drunk.

"It's unfortunate," says John Kimball, a New York partner at Blank Rome and specialist in maritime law who teaches admiralty law at New York University School of Law. "Here we have a decision that is in many ways a disappointment. The lower courts are now going to have to grapple with that."

Still, Howard, of the Maryland attorney general's office, is happy that the Ninth Circuit ruling stands: The states filed their amicus out of the very real fear that a ruling for Exxon, which was pressing for a ruling under the Amiable Nancy that no punitives should lie for Hazelwood's actions, would put shoreline states at a very real risk going forward.

And the court had some good news for environmentalists: It rejected Exxon's claim that the Clean Water Act "preempted" any claim for punitive damages under what is known as common law (meaning judge-made law).

The case is a win for Walter Dellinger of O'Melveny & Myers and a loss for Jeffrey Fisher, a professor at Stanford Law School who also argued Kennedy v. Louisiana, the child-rape case decided by the Court today.