Page "Associate Justice of the Supreme Court of the United States" Paragraph 4

Associate Justices have seniority by order of appointment, although the Chief Justice is always considered to be the most senior.

If two justices are appointed on the same day, the older is designated the senior Justice of the two.

Currently, the senior Associate Justice is Antonin Scalia.

By tradition, when the Justices are in conference deliberating the outcome of cases before the Court, the justices state their views in order of seniority.

If there is a knock at their conference room door, the junior justice ( who sits closest to the door ) must answer it.

The current junior justice is Elena Kagan.