A battle of footnotes in Monday’s Supreme Court global warming decision Massachusetts v. EPA makes it clearer than ever how crucial Justice Anthony Kennedy’s vote is to the outcome of big cases.

Justice John Paul Stevens, writing for the five-member majority, and Chief Justice John Roberts Jr., writing for the four dissenters, crossed swords over the meaning of Georgia v. Tennessee Copper Company, a completely obscure decision written by Justice Oliver Wendell Holmes Jr. just shy of a century ago. Holmes ruled that Georgia had standing to complain about the Tennessee company’s noxious emissions that resulted in a “wholesale destruction of forest, crops and orchards” in Georgia land just across the border from Tennessee.