With Congress having less than two days to pass a measure funding the U.S. government, a complete shutdown is becoming more of a possibility.

Lawmakers must extend funding by Friday night or the government will shut down early Saturday, CBS News reported.

But a government shutdown doesn’t mean every U.S. department or agency will close. Here’s the lowdown on what you need to know.

Which agencies will close?

Whether or not agencies and departments close or remain open depends on their level of importance. Workers at agencies and departments that are considered nonessential, such as museums and national parks, would stop working until a bill is passed, according to CNN.

Employees in “nonessential” departments would be placed on furlough — a temporary leave of employees due to special conditions, like not having money to fund operations. In the 2013 shutdown, about 850,000 employees were furloughed per day, according to the Office of Management and Budget.

What departments are essential? Who decides?

The typical course of action is that employees who perform “emergency work involving the safety of human life or the protection of property” remain on their jobs during a shutdown. Senior executives and legal advisors at each agency have traditionally decided who works and who doesn’t, per Politifact.

For example, the military is considered essential and would still report for duty, CNN reported.

Do federal employees, working or not working, still get paid?

Yes and no. Employees still get paid. But don’t expect the checks to come as quickly as they normally do. Marc Goldwein, senior policy director of the Committee for a Responsible Federal Budget, told FOX News that “an employee loses their paycheck at the time he or she needs it.”

Federal works typically receive back pay after they return to work, FOX reported. In previous shutdowns, everyone was paid retroactively, CNN reported.

How long does it last?

It depends on when an agreement is reached in Congress. The 2013 shutdown lasted 16 days.

But what would that mean for the country?

It could cost the country billions in lost economic activity. In 2013, the payroll cost of furloughed employees salaries was about $2 billion, according to the Office of Budget and Management.

If the shutdown happens on January 19, it will take effect on the one-year anniversary of President Trump’s inauguration.