4 ideas
Before the twentieth century, most societies did not run one unified currency. They ran several monies at the same time, and each served a different sphere of transactions. The integrated national currency is a recent historical exception, and treating it as the default distorts how earlier monetary systems are read.
Different monies in the same economy were complements, not competitors: each did a job the others could not. Low-value, high-frequency local exchange needed small, abundant units such as copper coins, cowries, or cloth strips. Long-distance and interregional settlement needed high-value, widely accepted media such as silver. Because they complemented each other, one money did not drive out another.
Local economies need a surge of transaction media at particular times, such as harvest and marketing seasons. The money that flows into the countryside then tends to stay there, held by many small users, rather than returning to issuers or merchants. As a result, local currency supply is hard to adjust, and the gap is often filled by locally generated substitutes such as private notes and token coins.
To understand a monetary system, ask which transactions and which users each money serves, rather than what the state declares as legal tender. This lens explains why some coins persisted far outside their issuing states: the Maria Theresa thaler, for example, circulated for generations around the Red Sea. Such money followed user demand for a trusted settlement medium, not sovereign authority.