As the country hurtles toward a shutdown this week, offices across the federal government are scrambling to finalize their contingency plans, knowing that many — but not all — operations will have to go dark.

A shutdown occurs when Congress cannot reach a deal to continue government funding by the time current funding lapses. One would begin at 12:01 a.m. Wednesday if Democrats and Republicans cannot come to an agreement.

In some parts of the federal government, there is an immediate impact. But not every federally funded initiative is at the mercy of Congress’s budgeting standoffs, and even some that are may be able to rely on cash reserves to get through a brief shutdown.

But if the two sides dig in, the consequences could be significant and lasting. Here is what to know.

What shuts down? What doesn’t?

The effects of a shutdown tend to unfold in stages. Some of the most visible expected closures are museums and monuments. Those planning a trip to a national park or the National Mall may find themselves greeted by locked doors and no entry signs if the shutdown lasts more than a few days. And federal workers will see their paychecks stop for the duration of a shutdown — even if they have to stay on the job.

Department to department, the impacts vary. The Postal Service, which is self-funded, and Social Security and Medicare benefits, which are considered “mandatory” spending, should keep on going. While most federal workers are likely to be furloughed, those designated as “excepted,” such as active-duty troops, air traffic controllers and federal law enforcement officers, stay on the job, even though they cannot be paid for that work until after the shutdown is over.