I mean, you just had to read into the second paragraph of gipper's Wiki link to show he has zero clue what's going on!
On November 28, 1961, President Kennedy halted sales of silver by the Treasury Department. Increasing demand for silver as an industrial metal had led to an increase in the market price of silver above the United States government's fixed price. This led to a decline in the government's excess silver reserves by over 80% during 1961. Kennedy also called upon Congress to phase out silver certificates in favor of Federal Reserve notes which, according to the
Associated Press at that time, were still backed by gold.
Kennedy halted the sales of silver because there was a run going on. Why would he do that if he wanted - allegedly - no silver-backed notes? Why would he want to keep a stock of silver in the first place? He could have just let the silver reserves run-off and voila! No silver-backed notes.
This is seems to escape the loons who have zero academic knowledge and zero practical experience in these things.
As that paragraph states, the currency was backed by gold. US dollars were convertible into gold by foreigners, though not be Americans. In the late 1960s and early 1970s, there was a run on gold as foreigners began to call away the US gold supply because the US was financing a large deficit to fund the Vietnam War. The budget deficit - as it often does - led to a trade deficit, and foreigners wanted to be paid in gold fearing the devaluation of the dollar. On August 15, 1971, Nixon suspended gold convertibility. But Kennedy didn't know this would happen. It makes zero sense to have a gold and silver backed currency, especially when there isn't enough silver in the country to back the currency in the first place.