Gold Standard
The Coinage Act of 1873 placed the United States on the gold standard, instead of a gold and silver standard. Due to citizen outrage, the U.S. Treasury also included between $2 and $4 million in silver bullion for coins.
Certificates
Since silver coins are significantly heavier than gold ones, the Treasury printed out promissory notes on paper that were legal tender redeemable in silver dollars. These were silver certificates.
The Great Depression
During the Great Depression, President Roosevelt quietly persuaded Congress to switch the U.S. back to the silver standard, as fluctuating gold prices were partially blamed for the poor economy.
Disappearing Silver Certificates
When silver certificates were presented to cash in on coins, the notes were shredded. Only if the treasury minted more coins did they draw up new notes.
End of Silver Certificates
After the 1970s, silver certificates could no longer be turned in for silver coins. They are still usable as legal tender, and even have a high collection value, but since silver was worth more by weight than by dollar amount, the Treasury would no longer hand out coins.