Now Treasury Secretary Yellen has joined the unfailing chorus, writing that "The U.S. has always paid its bills on time" and "The U.S. has never defaulted. Not once," and telling the Senate Banking Committee that if Congress does not raise the debt ceiling, "America would default for the first time in history."  

This is all simply wrong. If the United States government did default now, it would be the fifth time, not the first. There have been four explicit defaults on its debt before. These were:

The default on the U.S. government's demand notes in early 1862, caused by the Treasury's financial difficulties trying to pay for the Civil War. In response, the U. S. government took to printing pure paper money, or "greenbacks," which during the war fell to significant discounts against gold, depending particularly on the military fortunes of the Union armies.