According to SCOTBlog:
The question of how to count the votes of the justices to decide who won a Supreme Court case – and on what ground – when the court is splintered has baffled lower courts for years. The rule laid out in Marks v. United States purports to answer that question: “When a fragmented Court decides a case and no single rationale explaining the result enjoys the assent of five Justices, the holding of the Court may be viewed as that position taken by the Members who concurred in the judgment on the narrowest grounds.”
As a practical matter, the Marks rule has compounded rather than cured the confusion surrounding plurality precedent. Yet time after time when the Supreme Court has been confronted with an opportunity to clarify or abandon the Marks rule, it has failed to do so. More often than not, the court simply ignores the rule entirely, leaving lower courts in a hapless interpretative state each time the Supreme Court hands down a plurality decision. This could all change when the court decides Hughes v. United States, which is scheduled for argument on March 27.