The bearer could turn them in for the actual specie.

FORBES: Rethinking Money: The Rise Of Hayek's Private Competing Currencies

In the absence of convertibility into specie, there must be a monetary rule to anchor the nominal value of paper currency.

FORBES: The Limits Of Monetary Policy Call For Moral, Sound Money

RBS' officers hoarded the Bank of Scotland's bank notes, then presented them all at once in a surprise attack, demanding the gold and silver specie that backed them.

FORBES: Golden age redux