Only since 1869 have there consistently been nine justices appointed to the Supreme Court. Before that, Congress routinely changed the number of justices to achieve its own partisan political goals, resulting in as few as five Supreme Court justices required by law under John Adams to as many as 10 under Abraham Lincoln.

The U.S. Constitution is silent about how many justices should sit on the Supreme Court. In fact, the office of Chief Justice only exists because it’s mentioned in the Constitution under Senate rules for impeachment proceedings (“When the President of the United States is tried, the Chief Justice shall preside...”).

It’s Congress, not the Constitution, that decides the size of the Supreme Court, which it did for the first time under the Judiciary Act of 1789. When George Washington signed the Act into law, he set the number of Supreme Court justices at six.

Why six? Because Supreme Court justices in those days were also appointed to sit on federal circuit courts, of which there were 13 in 1789. Each circuit court would be presided over by three judges: one district court judge from the state and two Supreme Court justices.

“The justices had to spend almost the entire year traveling,” says Maeva Marcus, a research professor at the George Washington University Law School and director of its Institute for Constitutional History. “And the traveling conditions were horrendous.”

To limit the geographical area traveled by the justices, the Judiciary Act of 1789 divided the circuit courts into three regions: Eastern, Middle and Southern. The reason that the first Supreme Court had six justices was simple—so that two of them could preside in each of the three regions.

Marcus said that no one at the time quibbled about the fact that six is an even number, which leaves open the possibility of 3-3 split decisions.

“They never even thought about it, because all the judges were Federalists and they didn’t foresee great disagreement,” says Marcus. “Plus, you didn’t always have all six justices appearing at the Supreme Court for health and travel reasons.”

The Federalist's grip on power didn’t last, however, prompting the very first political controversy over Supreme Court nominations. In 1800, incumbent John Adams, a Federalist, lost the presidential election to Thomas Jefferson and the Democratic-Republicans.

In those days, the post-election “lame duck” session of Congress lasted until the following March, says Marcus, and Adams and his Federalists in Congress wanted to do everything in their power to deny Jefferson a Supreme Court pick.

While there’s a lot of controversy today around replacing a Supreme Court justice in an election year, Adams had no such qualms. In 1800, a month before the presidential election, Chief Justice Oliver Ellsworth resigned from the Court because of illness. Adams nominated and Congress confirmed Ellsworth’s successor, John Marshall, on February 4, 1801 during the lame duck session of Congress.

Adams and the Federalists then went a step further. They passed the Judiciary Act of 1801 which decreased the number of Supreme Court justices from six to five, further lowering the odds that Jefferson would get to nominate a new justice during his term in office.

In response, Jefferson and his new Congress quickly repealed the Judiciary Act of 1801, bringing the number of justices officially back to six. And since no justice had died in the interim, the number of seated justices never actually dropped to five.

By the start of the Civil War, the number of Supreme Court justices had increased to nine in order to cover additional circuit courts in the expanding American West. But Abraham Lincoln, upset over the Supreme Court’s 1857 decision in Dred Scott and wanting to cement an anti-slavery majority on the Court, added a 10th justice in 1863.

After the Civil War and Lincoln’s assassination, Congress clashed with Lincoln’s successor, Andrew Johnson, who was rapidly undoing the “Radical Republicans’” plan for Reconstruction. To limit Johnson’s power, Congress passed legislation in 1866 that cut the number of Supreme Court justices back to seven, all but assuring that Johnson wouldn’t have the opportunity to fill a vacant seat.

The last time Congress changed the number of Supreme Court justices was in 1869, again to meet a political end. Ulysses S. Grant was elected president in 1868 with the backing of the congressional Republicans who had hated Johnson. As a gift to Grant, Congress increased the number of justices from seven back to nine, and Grant gamely used those picks.

The Supreme Court had just ruled that paper money was unconstitutional, which would have “wreaked havoc” with the U.S. Treasury, says Marcus. But Grant and Congress quickly confirmed two new justices who reversed the Court’s decision in the earlier case, saving the Republicans from having to undo the nation’s entire system of legal tender.

In the 1930s, the Supreme Court issued a series of rulings that undercut some of Franklin D. Roosevelt’s New Deal legislation. FDR and his Justice Department responded with a proposed bill that would have allowed him to name six new Supreme Court justices to reach a grand total of 15.

Under the proposed legislation, all sitting justices older than 70 would be asked to resign. If any of them refused, FDR would be allowed to nominate an additional justice to the bench. Since six of the nine justices at the time were older than 70, that created the possibility of six new seats on the Supreme Court.

FDR’s plan, decried as “packing the court” with his political supporters, was shot down in the Senate by a vote of 70-20.