And yet nine is one of the worst numbers we could have come up with for a Supreme Court. The Constitution does not specify the size of the court. The first Supreme Court had six members, and, when it convened in 1790 at the Royal Exchange Building in New York, only two of those justices showed up. As of 1869, the court happened to have nine members because there were nine circuits, or federal court districts. And it has stayed that way ever since.

Our Supreme Court is the smallest of any major nation, and the result is that for periods of time we have a court of one, especially on hard cases — a single “swing vote” decides which way a 5-to-4 decision will tilt. It used to be that Justice Sandra Day O’Connor was the decider, then Justice Anthony M. Kennedy. Now it may be Chief Justice John G. Roberts Jr. Expanding the court my way would end that concentration of power. The court might split, but the deciding vote probably wouldn’t be the same justice again and again.

My proposal also avoids giving any president an advantage in nominating new justices. The increase in size would happen slowly: No president would be allowed to appoint more than two additional justices in a term. When the court was fully staffed at 19, natural turnover would start to regulate its makeup, with no one president likely to be able to push it in one direction or another, and yet with most presidents getting a chance to nominate newcomers to the court.

So why 19? Because that is roughly the size of our federal appellate courts, and such courts function well when sitting together, or en banc. I would add a rule that, each year, two of the high court justices would be dispatched to sit on lower courts, so they don’t lose touch with judging in the litigation brawl that precedes a case that rises to the Supreme Court. That would leave an active court of 17 justices. (If Congress did not want to circulate judges in the lower courts, it would expand the high court to 17 members, still big enough to spread the power and presidential prerogatives around.)