In an episode of the Netflix medieval series The Last Kingdom, the protagonist Uthred, trying to purchase a sword from a blacksmith in a town he is just passing by, is instantly asked “Do you have silver?”. In one scene, insignificant to the plot, the series creators neatly raised some fundamental questions in monetary economics, illustrating the relative use of credit and cash and the importance of finality.

• Cash transactions: direct finality, peer-to-peer (pure barter + commodity and paper money)
• Credit transactions: trade's another's debt, extinguishing prior debt by creating new (i.e., re-arrange)

Modern credit cards easily comes to mind: swiping that card settles the trade between the vendor and the customer who used the card only by creating two new (future) economic relations – a promise by the credit card company to transfer funds to the vendor, and a promise by the customer to pay the credit card company at the end of the month. The same features can be – and were – applied in many early societies; I give you some of my items, and you owe me; later I may transfer this “claim” to somebody else in the community in exchange for something I wanted, and instead of owing me, you owe them.

"Some of the difficulties of monetary economics are here quickly revealed. In order for credit to work, a sufficient level of trust, repeated dealings or enforcement mechanisms must exist. If one or more parties do not trust each other, the two are unlikely to trade again."

If we trust each other enough (or have enough repeat dealings and a system of keeping track of everyone’s debts), there is no need for cash. If there is need for cash, that means we do not trust each other (or can’t keep track/enforce debts), indicating the presence of “frictions” that make us reluctant to use credit at all.