WASHINGTON -- It's been 17 years since the federal government last faced a partial shutdown because Congress and the president couldn't agree on a spending bill.
A lot has changed in that time, leaving federal employees, citizens and even government decision-makers confused about what a shutdown would mean.
Every shutdown is different.
The politics that cause them are different. Because of technology and structural overhauls, the way the government functions has changed since 1996.
Much of what will happen is unknown.
Here's what we do know about the shutdown:
1. What causes a shutdown? Under the Constitution, Congress must pass laws to spend money. If Congress can't agree on a spending bill -- or if, in the case of the Clinton-era shutdowns, the president vetoes it -- the government does not have the legal authority to spend money.
2. What's a continuing resolution? Congress used to spend money by passing a budget first, then 12 separate appropriations bills. That process has broken down, and Congress uses a stopgap continuing resolution, or CR, that maintains spending at current levels for all or part of the year.
3. Why can't Congress agree? The Republican-controlled House has passed a spending bill that maintains spending levels but does not provide funding to implement the Affordable Care Act, or Obamacare. The Democratic Senate insists that the program be fully funded and that Congress pass what they call a "clean" CR.
4. What is a "clean" CR? A continuing resolution without policy changes.
5. Why is this happening now? The government runs on a fiscal year from Oct. 1 to Sept. 30. Shutdowns can happen at other times of the year when Congress passes a partial-year spending bill.
6. Could government agencies ignore the shutdown? Under a federal law known as the Anti-Deficiency Act, it can be a felony to spend taxpayer money without an appropriation from Congress.
7. When would a shutdown begin? When the fiscal year ends at midnight Monday. Most federal workers would report to work Tuesday, but unless they're deemed "essential," they would work no more than four hours on shutdown-related activities before being furloughed.
8. When would the shutdown end? Immediately after the president signs a spending bill. As a practical matter, it could be noon the following day before most government offices that were shut down would reopen their doors.
9. How many times has the government shut down in the past?@ Since 1977, there have been 17 shutdowns, according to the Congressional Research Service.
10. How long do shutdowns usually last? Most last no more than three days. Some last less than a day.
11. When was the longest shutdown in history? The longest was also the most recent: from Dec. 16, 1995, through Jan. 5, 1996.
12. Would this shutdown be different from those in the 1990s? Yes. When the 1995 shutdown started, Congress had already passed three of 13 appropriations bills. (They funded military construction, agriculture, and energy and water projects.) Also, more government services are automated.
THE DEBT LIMIT
13. What's the difference between a shutdown and a debt crisis? In a shutdown, the government lacks the legal authority to spend money on non-essential services. In a debt crisis, the government is mandated to spend money -- but doesn't have the legal authority to borrow the money to spend it.
14. Are the two related? Only by timing, which is somewhat coincidental.
15. When will the government run out of borrowing authority? Secretary of the Treasury Jacob Lew says it could come as soon as Oct. 17.
16. Has the United States ever defaulted on its debt before? No.
17. If the nation hits the debt limit, will government shut down? That's a big unknown question. The Treasury Department has said the most likely scenario is that it would delay payments, paying only those bills it can afford, using daily tax revenue.
18. Will I still get my mail? Yes. The U.S. Postal Service functions as an independent business unit.
19. Can I get a passport? Maybe, but hurry. The Department of State says it has some funds outside the annual congressional appropriation. "Consular operations domestically and overseas will remain 100 percent operational as long as there are sufficient fees to support operations," the department says.
20. Can I visit national parks? No. The National Park Service says day visitors will be told to leave immediately, and entrances will be closed.
21. What about campers already in the parks? They will be given two days to leave.
22. Will Washington museums be open? The Smithsonian, the National Zoo and the Holocaust Museum would all be closed. Private museums, such as the Newseum, the Spy Museum and Mount Vernon, would remain open. Rule of thumb: If it's usually free, it's probably closed.
23. What about the Kennedy Center for the Performing Arts? The Kennedy Center does receive an annual appropriation from Congress, but also runs on ticket revenue and endowment funds. The center expects to stay open through a shutdown.
24. What about the National Archives? All archives and most presidential libraries will be closed, unless they're operated by a private foundation -- as all pre-Herbert Hoover presidential museums are. The Federal Records Center Program, which supports other agencies, would continue to operate because it uses a revolving fund.
25. Will the District of Columbia shut down? The district does not have complete autonomy and relies on an appropriation from Congress to operate. So during the shutdowns in the 1990s, trash went uncollected, and many city departments closed. In a departure from past shutdowns, Mayor Vincent Gray has informed the Office of Management and Budget that he has deemed all city employees "essential." The district's own attorney general has declared the mayor's plan illegal.
26. Will the Patent and Trademark Office be open? Yes. The office can continue to operate off user fees and other funds for at least four weeks before having to shut down.
27. Would food safety inspections continue? Mostly. The Food Safety and Inspection Service would continue all safety-related activities. The Grain Inspection, Packers and Stockyards Administration would continue inspections to the extent they're paid by user fees, "but inability to investigate alleged violations could hamper corrective action in the long term and could have an immediate impact on members of industry." The Food and Drug Administration would limit its activities but continue to monitor recalls and conduct investigations.
28. Will the government still release economic data? Probably. The weekly unemployment claims number would still come out, and the September jobs report, due out Friday, probably will, too. The Department of Commerce reasons that some of its data is so economically sensitive that delaying it risks that it will be leaked.
29. Would the government continue to enforce wage and hour laws? The laws will still be in effect, but the Department of Labor's Wage and Hour Division would suspend operations.
30. Will disaster response be affected? No. However, all "non-disaster" grants -- such as state and local preparedness programs -- would be postponed, the Department of Homeland Security says.
31. Will e-Verify be affected? Yes. The government system to allow companies to voluntarily check the legal work status of its employees would be shut down.
32. Would a shutdown put the brakes on implementing the Affordable Care Act, or "Obamacare?" No. The state-run exchanges for the uninsured would open as scheduled Tuesday. "The marketplaces will be open on Tuesday, no matter what, even if there is a government shutdown," President Obama said Friday.
33. Why not? Like Social Security or Medicaid, Obamacare is a permanent entitlement that isn't subject to annual funding by Congress. "Many of the core parts of the health care law are funded through mandatory appropriations and wouldn't be affected," Gary Cohen, the Health and Human Services Department official overseeing the health care rollout, said last week.
34. Would seniors continue to get Social Security benefits? Yes. Social Security is a mandatory spending program, and the people who send those checks would continue to work under a legal doctrine called "necessary implication."
35. Can I apply for Social Security benefits, appeal a denial of benefits, change my address or sign up for direct deposit? Yes.
36. Can I get a new or replacement Social Security card, benefit verification statement or earnings record correction? No.
37. Would the government continue to pay unemployment benefits? Yes. The Employment and Training Administration "will continue to provide essential functions, as occurred during the shutdown of 1995," according to the Department of Labor contingency plan.
38. Will I be able to get food stamps? Yes. The Supplemental Nutrition Assistance Program (SNAP) is funded through the Recovery Act and from funds that don't expire for another year, the U.S. Department of Agriculture says.
39. What about WIC? No money would be available to pay the administrative costs of the Special Supplemental Nutrition Program for Women, Infants and Children. But because it's administered by states, there may be state funds available.
40. And the federal school lunch program? Schools are reimbursed for these costs on a monthly basis and are allowed to carry over funds from the previous fiscal year. The USDA expects most schools will be able to continue providing meals through October.
41. What will happen to veterans receiving compensation for service- or combat-related wounds and injuries? The Department of Veterans Affairs said if the shutdown continues into late October, it will run out of money for compensation and pension checks to more than 3.6 million veterans who rely on the money to support themselves.
42. Can I still get a federally backed loan? Maybe not. "Federal loans for rural communities, small business owners, families buying a home will be frozen," President Obama said Friday.
43. Does that mean I can't get an FHA mortgage? No. The Federal Housing Administration says it "will endorse new loans under current multi-year appropriation authority in order to support the health and stability of the U.S. mortgage market."
44. Does that mean I can't get a VA mortgage? No. The Department of Veterans Affairs says loans are funded via user fees and should continue. However, during the last shutdown, "loan Guaranty certificates of eligibility and certificates of reasonable value were delayed."
45. Will deceased veterans still be able to get a burial benefit? Yes. Burial benefits, headstones and death notices will still be available.
46. Would the IRS continue to collect taxes? Yes. All payments would be processed. More than 12 million people have requested an extension on their 2012 taxes, which expires Oct. 15.
47. Will my refund be delayed? Possibly, especially if the taxpayer files a paper return.
48. What about taxpayer assistance? Walk-in assistance centers and telephone hotlines would be closed.
49. I'm being audited by the IRS. Would a shutdown affect me? Yes. The IRS will suspend all audit activities.
50. How many federal employees would be furloughed? The government has not given an official estimate.
51. Does anyone have a guess? J. David Cox, president of the American Federation of Government employees, said he expects the number will be 800,000 to 1 million, out of 2.1 million federal employees. That's consistent with a USA TODAY analysis of 2011 shutdown contingency plans, which found that 59 percent of non-defense government employees would continue to work.
52. Why do some federal employees continue to work during a shutdown? The law -- or at least, the Justice Department's interpretation of it -- contains exemptions for several classes of employees: The biggest exemption is for employees necessary to protect public health, safety or property. But property could include government data, ongoing research experiments or other intangibles. Political appointees are exempt because they cannot be placed on leave by law. Employees necessary for the president to carry out his constitutional responsibilities are exempt. Finally, employees whose salaries are paid from sources outside an annual spending bill can still get paid and report to work.
53. Who decides which employees work and which go home? Each agency is responsible for coming up with its own contingency plan, based on guidance from the Office of Management and Budget and the Office of Personnel Management. Those plans are then sent to the White House for review.
54. Would the president be paid during a shutdown? Yes. The president's $400,000 salary is mandatory spending. If furloughs begin to affect the government's ability to process payroll, his paycheck could be delayed.
55. What about White House staff? Some high-ranking presidential appointees are exempt from the Annual and Sick Leave Act of 1951, which means they can essentially be made to work unpaid overtime. Also, any employee necessary for the president to carry out his constitutional duties would be exempt.
56. And the president's personal aides? The White House has 90 staffers who work in the residence. During a shutdown, 15 of them would stay on the job.
57. Would Congress continue to be paid during a shutdown? Yes. The 27th Amendment to the Constitution, ratified in 1992, holds 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." Intended to prevent Congress from voting itself a raise, it also protects members from a pay cut.
58. What about congressional staff? Like other federal employees, they would be deemed essential or non-essential. Essential staff would include those necessary to carry out constitutional responsibilities, such as the parliamentarians, or for protection of members, such as the sergeants-at-arms. Staff of the appropriations committees may also be needed to write the law that would end the shutdown.
59. Would active-duty military be furloughed? No. All active-duty military are essential and should report as scheduled Tuesday, the Department of Defense said Friday.
60. Will civilian defense workers be furloughed? About half of them, or about 400,000, will be sent home, according to the Defense Department's contingency plan.
61. Would active-duty military be paid during a shutdown? If a shutdown lasts longer than a week, the Pentagon might not be able to process its payroll in time for the Oct. 15 paychecks, Defense Department Comptroller Robert Hale said Friday. The House passed a separate bill early Sunday that would appropriate money for active-duty and reserve paychecks regardless of the shutdown -- and also pay for support services to make sure they get paid. That bill passed the House 422-0, but still must go to the Senate.
62. Could federal employees simply volunteer their services? No. A 19th-century federal law forbids volunteers because the government doesn't want them filing claims for back pay after the shutdown is over, according to a legal analysis by Washington attorney Raymond Natter.
63. Would federal employees get paid retroactively, even if they didn't work? Maybe. Congress granted retroactive pay to furloughed workers after the shutdowns of the mid-1990s, but that wouldn't necessarily happen again. "I believe this time is going to be much different. This is a much different Congress than the 1995 Congress," said Cox, federal employee union president. "I'm not sure that they'd even want to go back and pay the people who worked."
THE LONG TERM
64. How much money would a shutdown save taxpayers? Most likely, it wouldn't. The Committee for a Responsible Federal Budget says shutdowns cost money in terms of contingency planning, lost user fees and back pay. A government estimate after the shutdown in 1995-96 estimated its cost at $1.4 billion.
65. What effect would a shutdown have on the economy? Economists say even a short shutdown -- of three or four days -- would begin to shave decimal points off economic growth. A sustained shutdown of three or four weeks "would do significant economic damage," economist Mark Zandi told USA TODAY.
66. What about the stock market? The Standard & Poor's 500 fell 3.7 percent during the 1995-96 government shutdown, according to S&P Capital IQ. Stocks quickly rebounded after the government got back to work, rising 10.5% the month after the shutdown ended.
67. What's a furlough? Unpaid leave. Or, in the official definition from the Office of Personnel Management: "A furlough is the placing of an employee in a temporary non-duty, nonpay status because of lack of work or funds, or other nondisciplinary reasons."Related: How the shutdown may impact
68. Will Congress cancel any scheduled hearings? Yes, but with exceptions. Before a threatened shutdown in 2011, the chairman of the House Oversight Committee tweeted that oversight is a constitutional responsibility and should continue through a shutdown. "If gov't shuts down, we won't. I believe those who choose to come into work fall under my constitutional arm," Rep, Darrell Issa, R-Calif., said. This week, the committee has scheduled two hearings on Tuesday and Wednesday, although a third hearing on state marijuana laws has been canceled.
69. Will members of Congress close their district offices? Guidance from the House Administration Committee says that only staffers who are needed for members to carry out their constitutional responsibilities -- writing laws, voting and communicating with constituents -- will continue to work. But members are interpreting that guidance differently. Rep. Todd Young, R-Ind., is closing district offices. Rep. Kyrsten Sinema, D-Ariz., is expanding office hours.
70. Will federal courts be closed? The federal judiciary would remain open for at least 10 days, according to the Administrative Office of the U.S. Courts. All scheduled court dates will continue, and federal employees preparing for trials are exempted. If a shutdown lasts through mid-October, the courts would reassess the situation.
71. What about the Supreme Court? It's open at least through Oct. 4 -- and remains one of the few public buildings in D.C. still open to visitors. Oral arguments for the new fall term begin next Monday and are likely to go forward as scheduled.
72. What about the U.S. Tax Court? Closed, although the court accepts filings until noon Tuesday. The Tax Court, which hears disputes between the IRS and taxpayers, is technically in the legislative branch of government. Also, IRS tax audits would be suspended. Due dates would be extended by the length of the shutdown, although new filings must still be postmarked within 90 days of an IRS notice of deficiency.
73. Would there be longer lines at the airport? Initially, no. The Transportation Security Administration says air passenger screening is "necessary for safety of life and protection of property" and all screeners would be exempted. Training -- including ongoing training on privacy and civil liberties -- would cease, however.
74. What about federal air marshals? They will continue to work, although no one would know if they didn't because the number of air marshals is classified.
75. What about Federal Aviation Administration inspectors? "Aviation safety inspectors will be off the job at midnight," Mike Perrone, president of the Professional Aviation Safety Specialists union, said before the shutdown. That wouldn't immediately delay flights, but could result in grounded planes as inspections become due. Also, long-term projects like the Dreamliner certification would be delayed.
76. Will Amtrak trains continue to run? Yes, for now. A statement from the federally subsidized rail service said the trains would run during a "short-term federal government shutdown" in "the coming days or weeks."
77. Would the Consumer Product Safety Commission continue to issue product recalls? Yes, if the products "create an immediate threat to the safety of human life."
78. Will Freedom of Information Act requests be processed? No.
79. Will NASA continue to operate? "NASA will shut down almost entirely," President Obama said Monday. There are exceptions. Two American astronauts -- Karen L. Nyberg and Mike Hopkins -- are aboard the International Space Station with three Russians and an Italian. Mission Control in Houston will continue to support them. Popular NASA websites would shut down.
80. What about the National Zoo's Giant Panda Cam? "None of our live animal cams will broadcast," said the zoo, which is run by the Smithsonian Institution. Animals will continue to be fully cared for.
81. Would a shutdown bring an increase in Washington's rat population? No. Some commentators have said Washington, D.C., rats would be one of the few clear winners in a shutdown, on the theory that they would feast on trash that isn't picked up. The District of Columbia's Department of Health confirms there was an increase in the rat population during the last shutdown, in 1995-96. But this time around, Mayor Vincent Gray has deemed all city employees "essential," including sanitation workers.
82. Will student loans be impacted? Not in the short term. The 14 million students receiving direct federal student loans and Pell grants would continue to receive them, "dependent on the length of the lapse," the Department of Education says. A "skeletal" staff would be retained to run those programs.
83. How does the shutdown affect the schools and kids in special programs? It shouldn't. Schools are run by states with federal aid for programs like special education. Congress has already appropriated $22 billion in state educational aid, but it hasn't been contractually awarded, or obligated, by the Department of Education. Those obligations began Tuesday. The Department of Education said delaying these funds could "significantly damage state and local program operations," and so will continue to employ staffers to process those grants.
84. What about Head Start? About 20 of the nation's 1,600 Head Start centers for preschoolers would have to close immediately, because their grants expire Tuesday. Most are in Florida. In the Tallahassee area, that would hit nearly 400 children, their families and 100 staffers.
85. Will federal retirees continue to get their pensions and health care? Yes. Those benefits are paid through trust funds that federal employees pay into. "There are sufficient amounts in these trust funds to operate them throughout the duration of any lapse in appropriations," the Office of Personnel Management says.
86. Will veterans still be able to get health care? Yes. The Department of Veterans Affairs has already been appropriated money from Congress. "This means that our hospitals, clinics and other health services will remain open," spokeswoman Victoria Dillon said.
87. What about other veterans' benefits? Pension, education and vocational rehabilitation payments will continue at least through late October, Dillon said. If the shutdown lasted longer, the programs could be suspended.
88. Can I still get a Small Business Administration loan? No. The only program that the SBA will continue to operate is the Disaster Loan Program.
89. I have a government benefit check I haven't cashed. Is it still good? Yes. The shutdown applies only to the government's ability to make new payments.
90. Why aren't military salaries considered mandatory spending? "Mandatory" and "discretionary" are congressional terms that apply to how Congress spends the money, but has nothing to do with the importance of the spending. Mandatory spending includes programs like Social Security and Medicare, which have separate sources of funding and run automatically. Discretionary spending includes anything that Congress spends year-to-year with tax revenue and borrowed money. On Monday, President Obama signed a law ensuring active-duty and reserve military would be paid during a shutdown.
91. Will military health care be impacted? Military hospitals and health and dental clinics could be limited to treatment for active-duty servicemembers only, or for acute care and emergencies for other patients. Tricare coverage is not expected to be immediately affected because contracts for the Tricare providers are not limited to a single year.
92. Will government websites shut down? Most websites continued to operate Tuesday morning, though some agencies, like the Federal Trade Commission and the National Park Service, began replacing their front pages with "splash" pages. Some agencies may leave their sites up, but there won't be tech support to fix them if they break. Websites necessary for critical government functions will still operate, but won't be updated. The IRS website is one of those.
93. What is one of the most bizarre government program canceled by the government shutdown? The Gateway National Recreation Area in Sandy Hook, N.J., has furloughed its poison ivy-eating goats.
Gregory Korte, USA TODAY
Contributing: Adam Shell, Tim Mullaney in New York, Gregg Zoroya in McLean, Va., Raju Chebium of Gannett News Service; Rick Maze and Andrew Tilghman of Military Times; Surae Chinn of WUSA-TV in Washington; and the Associated Press.