Silver

SILVER STANDARD is a monetary standard under which the currency unit is defined by a stated quantity of silver.
The silver standard is believed to date back to ancient Greece, where silver was the first metal used as a measure of currency. After the fall of the Roman Empire, the adoption of the silver standard was widespread and included its use in China, India, Bohemia, Great Britain, and the United States.
On June 4th, 1963, President Kennedy signed Executive Order 11110.
In short, it was an order for the Treasury to issue certificates backed by silver bullion, as well as to mint silver dollars that would be considered money.
These silver certificates took the form of dollars for the most part, and the notes would say that whatever the denomination on the bill was, it would be repaid in silver "to the bearer on demand." Simply put: if you had $1, it was worth $1 in silver.
Unfortunately, the executive order didn't stand the test of time.
That November, President Kennedy was assassinated in Dallas, sparking conspiracy theories that the Federal Reserve had a hand in his death.