What's interesting about this is "Skymiles" are a form of private currency. Yes, there are many private currencies in use.
Banknotes also used to be private money. Each bank issued their own. The government eventually made that illegal, but banks still issue private currencies in the form of:
1. personal checks
2. cashier's checks
3. traveler's checks (though I think Amex stopped printing them)
I think you're conflating currency with interfaces to the financial system. The currency is the medium of exchange: the dollar itself. A debit card is merely a tool to transact using dollars.
I'm not conflating things. They're currency. I talked about this with my dad, who was head of the finance department at a college. He had a degree in economics from MIT and an MBA from Hahvahd.
> A debit card is merely a tool to transact using dollars.
And banknotes were just a convenient tool to transact gold that was on deposit in the bank vault.
Banknotes also used to be private money. Each bank issued their own. The government eventually made that illegal, but banks still issue private currencies in the form of:
1. personal checks
2. cashier's checks
3. traveler's checks (though I think Amex stopped printing them)
4. credit cards
5. debit cards