A federal government shutdown means that thousands of government employees — from Washington bureaucrats to park rangers — go without paychecks until the standoff in Washington resolves.

But there are 535 federal employees who still get paid: members of Congress.

Unlike other federal employees, senators and House members continue to get paid through a government shutdown. That’s because their salaries are literally written into the Constitution. Article 1, Section 6 specifies, “The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States.”

What’s more, the 27th Amendment actually prevents Congress from changing its own salaries between elections. It states that “No law varying the compensation for the services of the Senators and Representatives shall take effect, until an election of Representatives shall have intervened.”

The Constitution does not, however, protect the salaries of congressional staff members or any other federal employees. As Vox’s Tara Golshan and Dylan Matthews explain, the rest of the government is divided into two groups — one that continues to work, and one that is furloughed and told to stay home:

Federal employees are split into “essential” and “nonessential” groups. Nonessential employees receive furloughs: They stop getting paid and are off work until the shutdown is resolved. Essential workers also stop getting paid, but they still have to work. Usually when a shutdown is over, federal employees are paid back the salaries they went without.

Some members of Congress have already promised to forgo their salaries during the shutdown, possibly donating them to charity instead. Rep. Mark Walker (R-NC) says he’ll be “refusing” his salary.

As long as 100,000 active duty service men and women based in NC are defending our freedom with no pay, the very least I can do is lead by example with my letter to the CAO of Congress refusing my salary as well. #SchumerShutdown pic.twitter.com/JlUmWI9JBW — Rep. Mark Walker (@RepMarkWalker) January 21, 2018

The Washington Post estimates that 248 Congress members refused their salaries or donated them to charity during the 2013 shutdown, which lasted 16 days.

Late last week, some moderate Democrats, led by Sen. Claire McCaskill (D-MO), introduced a bill that would bar legislators from getting paid during a shutdown. It’s not clear how this would interact with the 27th Amendment — which seems to pretty clearly state that such laws cannot be passed between elections.