STILL FRAGILE: The Federal Reserve extended a program set up earlier this year to ease financial strains from the European debt crisis.

HOW IT WORKS: The Fed lends dollars to the European Central Bank as well as other central banks in exchange for their currencies. The central banks can lend those dollars out to banks in their home countries.

DOLLARS IN DEMAND: European banks need dollars to lend to companies across the continent. European companies that have operations in the U.S. pay their employees in dollars and buy raw materials with the U.S. currency.