Comment: Historically...

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In reply to comment: Ok, but then what would back (see in situ)

Historically...

...each commercial bank issued its own banknotes backed by gold or silver. That is, a depositor goes to ABC bank, deposits $100 in gold coins, and receives $100 in ABC banknotes.

"Alas! I believe in the virtue of birds. And it only takes a feather for me to die laughing."