Here is the last (for now) in my short series on Modernisms in Fantasy Gaming.
These are modern assumptions that players may bring into play, despite the fact that they post-date the rough time period being portrayed or require social organization and technology that is beyond that in the game. As such, they can trip up the players when they hit the different reality of games.
Universal, Freely Exchanged Currency
GURPS DF has copper pieces ($1), silver pieces ($4) and gold pieces ($80). DF Felltower has its own devalued set. AD&D has copper, silver, electrum, gold, platinum, Rolemaster has coins from bronze to gold going by tens, and Dragonlance even has steel pieces (Hah*).
Generally, though, those coins are the same everywhere. It's rare for places to have extra coins. It's exceedingly rare outside of a few old school modules to have places that have their own currency and laws about passing foreign coins.
And yet historically, coinage varied wildly from place to place. Coins were often debased and devalued and nicked and trimmed. Even a handful of the same coins may not really be worth the same amount as each other, never mind another handful of the same type of coins. A gold piece might not be worth 1 gp. Moneychangers would convert foreign coins to local coins, taking a cut in return for engaging in such trade. And for sufficiently odd currency, it's possible that it wouldn't be accepted at all due to the difficulty of exchanging it (such as Yap stones, shell coins, paper currency prior to its local adoption.)
This is one games have largely bowed to - the idea that your coins are good everywhere. But if a GM uses local coins and rules about local usage - and required coins changed at moneychangers - it can cause a lot of difficulty. I've run a few games were the players needed to convert all foreign coins to local coinage. The result has ranged from campaign-ending violent refusal by the PCs to merely committing large-scale fraud by not converting to arguing a lot about getting ripped off.
And to be fair, the modern world has exchangeable currency but no really universal one - I keep yen handy despite the reach of the dollar, and converted to Euros last time I was in Europe. Yet in games, everyone expect a "gold piece" to be worth a uniform amount and be universally accepted.
** Hahahahahahahahahah. Okay, sorry. They make no sense.