At Risk: Currency Privilege of the Dollar
Not all government debt is created equal. Some governments get a much better deal than others, and no one gets a better deal than the United States.
The United States borrows in its own currency, and it borrows at extremely low interest rates. It also borrows under its own laws, an often overlooked advantage. Such a situation makes default or at least involuntary default impossible because the government can print dollars if need be. The value of the dollars it repays may be less than the value of the dollars it borrows, but that is a risk the lenders accept. The United States could change its laws, but it is trusted not to abuse that right.
The perils of not having that flexibility became clear 80 years ago.
In 1933, when Franklin D. Roosevelt became president, the world was in the Great Depression and many countries were devaluing their currencies in a desperate attempt to stimulate exports and growth. That left the United States at a disadvantage, and one of the first things the president did was to persuade Congress to devalue the dollar.
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http://www.nytimes.com/2013/10/11/business/at-risk-the-dollars-privilege-as-a-reserve-currency.html?_r=0