Why does the Supreme Court consist of nine justices?

Certain aspects of the U.S. government—such as presidential terms and post offices—were included in the original Constitution after lengthy debates among the Founding Fathers. However, the number of Supreme Court justices was not one of those matters.

The Constitution did create the Supreme Court and specified that the president should appoint its justices, mentioning that a “Chief Justice shall preside” in the event of presidential impeachment. As the details were left to Congress, they enacted the Judiciary Act of 1789, which established a full court system and stated that the Supreme Court would have one chief justice and five associate justices. History.com notes that the decision was made to have six justices so they could oversee federal circuit courts across the country, one of which was in each state. Due to the travel difficulties of the time, Congress sought to limit the jurisdiction of each justice by dividing the courts into three regions and assigning two justices per region.

Maeva Marcus, director of the Institute for Constitutional History at George Washington University Law School, notes that the even number of justices was not a concern. “They never even thought about it, because all the judges were Federalists and they didn’t foresee any major conflicts,” she told History.com. “Besides, not all six justices would be present at the Supreme Court due to health and travel constraints.”

Over the next 80 years, the number of Supreme Court justices fluctuated for two main reasons: the creation of federal circuit courts and the political motivations of presidents. John Adams and his Federalist-controlled Congress reduced the number to five with the Judiciary Act of 1801, aiming to prevent Democratic-Republican Thomas Jefferson from appointing a justice after taking office that year. In 1802, Jefferson’s Congress passed another judicial act, increasing the number of justices to six, and by 1807, it was raised to seven after the creation of another circuit court.

As the nation expanded in the early 19th century, Congress added two new circuit courts—and consequently, two additional Supreme Court seats—during Andrew Jackson’s presidency in 1837. Later, Republican Abraham Lincoln briefly increased the number of justices to 10 to secure an extra abolitionist vote, but Congress reduced it back to seven in 1866 to prevent Andrew Johnson from appointing Democrats. When Republican Ulysses S. Grant succeeded Johnson, Congress restored the number to nine, where it has remained since.

In 1911, Congress eliminated circuit courts entirely, meaning the number of Supreme Court justices no longer depended on their expansion (although each justice still oversees a region to assist with occasional tasks). As for presidents altering the number to suit their objectives, this is now referred to as “packing the court.” When Franklin D. Roosevelt attempted to increase the number to 15 in the 1930s to push his New Deal policies through the Supreme Court, the Senate rejected the proposal by a decisive 70 to 20 vote.

In brief, the number of justices on the Supreme Court fluctuated significantly in the early years of the United States, not only due to the country’s expansion but also because the federal government was still experimenting with its system of checks and balances. While presidents continue to appoint justices aligned with their political party, the idea that the Supreme Court should remain, at least ideologically, impartial has become ingrained. If Congress and the president continued to change the number of justices whenever they pleased, it would undermine this principle.

Steve Vladeck, a professor at the University of Texas School of Law, wrote for NBC News, "If Congress increases the size of the Supreme Court for clearly partisan reasons, it would reinforce the notion that the justices are merely politicians in robes, and that the court itself is just another powerful tool for exercising political power." He continued, "The fact that Congress has not altered the court’s size in 150 years is a strong indication of how deeply the norm of nine justices has taken root, and how concerned different political groups have been over the years about safeguarding the court’s integrity."