Q. What about the rest of the world? Will other countries continue to invest in the United States? Would a default send investors to the safety of other currencies?

A. There are already indications that this is beginning to happen. Switzerland’s franc strengthened to a record high against the dollar this week, as concern about the debt limit in the United States and worries about the euro, given the Greek crisis, sent investors looking for safety. Some bond funds are already moving to invest in bonds from Canada, Mexico and China. And there are concerns about what would happen if America’s foreign creditors, led by China, were to try to dump some of their American debt. In the last decade, foreign money has poured into the United States, creating so much demand for Treasury bills that it has kept the United States’ interest rate low. This month, the authorities in Beijing expressed concern about the debt standoff in the United States. “We hope that the U.S. government adopts responsible policies and measures to guarantee the interests of investors,” said Hong Lei, a Foreign Ministry spokesman.

Q. How many times has the debt ceiling been raised, and by whom?

A. It has been a bipartisan exercise. By the Treasury Department’s count, Congress has acted 78 times since 1960 to raise, extend or alter the definition of the debt limit — 49 times under Republican presidents, and 29 times under Democratic presidents. The Obama administration has taken pains to note that President Ronald Reagan, a hero to many Republicans in Congress, raised the debt limit. In a letter on the debt ceiling last month to Republicans in the Senate, Treasury Secretary Timothy F. Geithner quoted a letter Mr. Reagan wrote a generation ago, urging Congress to increase the debt limit. “The full consequences of a default — or even the serious prospect of default — by the United States are impossible to predict and awesome to contemplate,” he quoted Mr. Reagan as writing. “Denigration of the full faith and credit of the United States would have substantial effects on the domestic financial markets and on the value of the dollar in exchange markets. The Nation can ill afford to allow such a result.”

Q. How has the debt risen this high, and how much are we paying in interest as a nation?

A. The United States has not always operated with such a large debt. After financing World War II with substantial borrowing, the outstanding debt held pretty stable for the next 25 years, going up to $283 billion in 1970 from $242 billion in 1946. But over the last 30 years, the overall debt has increased under every president — with the biggest increase under President George W. Bush, who cut taxes, added a drug benefit to Medicare and fought two wars. As the debt has grown, so have the country’s interest payments. In 2003, for instance, the government paid about $150 billion in interest costs; this year it is estimated to be upward of $200 billion. These interest payments are taking up more federal spending now than federal outlays on education, transportation and housing and urban development combined. Though the interest costs are substantial, they have remained lower than some economists predicted because the world has continued to lend money to the United States at very low interest rates, even as the nation’s debt has grown.

Q. Has what is going on in Washington already hurt the economy and the reputation of the United States in financial markets around the world?

A. Stocks fell steeply on Wednesday on worries that the United States could default or see its credit rating cut, and Treasury market analysts and traders are already saying that the credibility of the United States has been damaged. Mark Zandi, the chief economist of Moody’s Analytics, said last week: “Our aura is diminished. You know people really view the U.S. as the AAA, the gold standard, and I think we’re tarnishing that.” Treasury bonds have always been considered to be virtually risk-free, and that is why many investors — in the United States and abroad — hold them and many companies use them to back up other investments. If their security is questioned, investors may shift away from them. The caveat, though, is that there are few safe places today for investors to put their cash. So some traders say that investors may see few alternatives, and opt to stay put.

Q. Has the United States defaulted on its debt before?

A. Technically, yes. In 1979, as Congress was considering raising the debt limit, negotiations ran down to the wire. The Treasury Department had what it called technological glitches, and it was late paying a relatively small number of its Treasury notes. This amounted to a technical default, not a permanent one, because the note holders were eventually paid in full. Some finance professors who have studied the incident say it led to higher interest rates.