In Ironbound, trade and physical mass are anchored by a universal standard coin: the Copper Penny (cp)1). For the sake of convenience, the silver penny (sp) is also occasionally used; there are ten copper pence to the silver penny.
While political borders shift and local rulers mint their own regional currencies, commerce across the realm relies on a baseline standard maintained not through brute force, but through sheer ubiquity and institutional trust.
Of course, your own fantasy campaign may well establish its own coinage and rules.
The copper penny serves a dual mechanical purpose in the rules:
The absolute authority of the standard penny is preserved by a widespread, pan-regional organization (such as a dominant mercantile guild, a ubiquitous temple network, or a historic banking cartel) with a presence in nearly every major settlement and trade hub.
Third-party publishers and Game Masters should define the specific organization, deity, or merchant syndicate that holds this role within their respective campaigns.