The impasse over President Donald Trump’s campaign promise to build a wall along the U.S.-Mexico border now threatens to produce the longest U.S. government shutdown ever. Though some departments are operating, and others are staffed by “essential” employees, the effects of the shutdown are being felt well beyond Washington, with neither Trump nor opposition Democrats showing any signs of backing down.

1. Why is there a government shutdown?

The U.S. government runs on 12 appropriations bills passed each year by Congress and signed by the president. In fiscal years like this one, when all 12 bills aren’t adopted by the Oct. 1 start of the fiscal year (only five were completed on time), Congress and the president keep the machinery of government humming by passing short-term extensions. They followed that process this time, but then Trump demanded that any further extension include $5.7 billion for his border wall.

2. How much of the government is closed?

Nine federal departments and agencies such as the Federal Aviation Administration, Environmental Protection Agency and Securities and Exchange Commission have been closed since Dec. 22, when their funding ran out. Other pieces of the government, notably the Defense Department, are funded because Congress and Trump had managed to reach agreement on their 2019 appropriations. Still others, like the U.S. Postal Service and U.S. Federal Reserve, have funding streams separate from what Congress provides. In closed departments and agencies, only employees deemed “essential” report to work, and they won’t be paid until the shutdown is over.

3. Who is essential?

Generally speaking, government workers in law enforcement and public safety continue to work — so air traffic control, medical care of veterans and federal criminal investigations are moving forward during the shutdown. But defining “essential” is more art than science, with individual departments — and the political appointees who run them — having a say over who comes to work and who stays home. In theory at least, a federal employee who works during a shutdown, but isn’t supposed to, could face fines or a prison term under what’s called the Antideficiency Act.

4. What’s been the impact so far?

The Food and Drug Administration isn’t doing some routine food safety inspections and might run out of funds to review new drugs. The SEC can’t approve initial public offerings. Airlines can’t get permission to expand fleets. With mortgage lenders unable to verify borrowers’ incomes, home closings are being delayed. Some fishing boats in Alaska are stuck in dock, in need of federal permits and inspections. U.S. airport screeners, though declared essential, won’t be getting paychecks and are calling in sick in larger-than-normal numbers. Trash is piling up in national parks, which are open but unstaffed. The Agriculture Department has found funds to provide food stamp assistance through February but would run out after that. The Commodity Futures Trading Commission has stopped releasing its weekly Commitments of Traders reports. Hundreds of thousands of federal workers will have to make due without their paychecks.

5. Will federal employees eventually get paid?

Probably. When a shutdown happens, most federal employees are placed on unpaid furlough. Though there “appears to be no guarantee” that they will eventually be paid, in practice they always have been, retroactively, via legislation passed by Congress, according to the Congressional Research Service.

6. What will this mean for the U.S. economy?

That depends on how long a shutdown lasts. Kevin Hassett, chairman of the White House Council of Economic Advisers, said the shutdown will cut U.S. economic output by about 0.1 percent every two weeks. That’s about in line with an estimate by Bloomberg economists that a government shutdown that lasted two and a half weeks in 2013 subtracted 0.30 percentage point from quarterly gross domestic product.

7. How might this end?

Trump could give in and agree to fund the government without progress on his wall, or congressional Democrats could give in and pledge money for the project. There’s room for compromise between the $1.6 billion Democrats previously offered for border security and the $5.7 billion Trump wants specifically for a wall; plus, Trump seems willing to expand his definition of an acceptable “wall.” Trump could try to sweeten the deal for Democrats, perhaps by offering legal protections for the young, undocumented immigrants known as “Dreamers.” Then there’s Trump’s talk of declaring a national emergency under which he could shift military construction funds to build the border wall, removing the issue from budget talks. That would be an extraordinary use of a presidential prerogative usually reserved for prosecuting a war or restricting trade and transactions with a foreign adversary, and would likely draw legal challenges.

8. How many times has the U.S. government shut down?

There have been 13 shutdowns since 1981, ranging from one to 21 days, including a three-day one last January. (Before 1981, agencies operated mostly as normal during funding gaps, their expenses covered retroactively once a deal was reached.) The 21-day shutdown, in December 1995 and January 1996, was a famous budget impasse that pitted President Bill Clinton, a Democrat, and the Republican House speaker, Newt Gingrich. Shutdowns over spending disagreements are different (and less grave) than what would happen if the U.S. breached its debt ceiling and defaulted on some of its obligations. That’s never happened.