Since 1971, U.S. citizens have been able to utilize Federal Reserve notes as the only form of money, and for the first time had no currency with any gold or silver backing.
This is where you get the saying that U.S. dollars are backed by the “full faith and credit” of the U.S. government. In other words, Nixon implied "take our paper dollars or don’t".
The U.S. at this time was a world super power having been victorious in WWII and there really wasn’t much anyone could do about the decision by the U.S. government to abandon metal backing.
What does a dollar or Federal Reserve note represent now that gold and silver no longer back any of the currency printed in the U.S.?
A dollar bill used to say,
This note is legal tender for all debts, public and private, and is redeemable in lawful money at the United States Treasury or at any Federal Reserve Bank.
Look at a dollar bill today. It simply says:
This note is legal tender for all debts, public and private.
In other words, you can’t redeem it for “lawful money.
Guess what folks? A dollar bill is not lawful money, but rather “
legal tender.”
http://seekingalpha.com/article/145722-what-really-backs-the-u-s-dollar