Most everything at the Supreme Court is bound by tradition - and so is seating. The chief justice always sits in the middle of the court's angled mahogony bench. The most senior associate justice sits to his right. The next longest-serving justice sits to his left. And the seniority pattern continues, right to left, with the most junior justice always at the farthest end of the bench to the chief's left.
Robert Barnes and Patterson Clark - The Washington Post