system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did so many different currencies all come to use the dollar sign ($)?

A lot of other currencies seem to have symbols specific only to themselves (€, £), so I have been wondering how it is that the dollar sign is common to so many different currencies worldwide. Off the top of my head, I can think of pesos, USD, CAD, and AUD. I'm also interested in why more than one country calls their currency "dollars" even though USD, CAD, and AUD are very different and have different values in the international exchange.
gpt
It wasn't really until the 1648 and the signing of the Treaty of Westphalia that the modern concept of a nation state with fixed borders really started. Before then unless you were an island like the UK then your borders shifted as kings died and neighbouring countries extorted taxes from whatever territories they could.

So without different countries money was just some token of value that people only trusted to some extent. If it was a gold or silver coin then it didn't really matter which king's face was on it, it still had value.

The town of Jáchymov in Czechia had a silver mine and for centuries minted a coin from this silver, the German name for Jachvmov is Joachimsthal (Saint Joachim's Valley), the coins were known as Joachimsthallers, or thallers for short which would be roughly pronounced as 'dollar'.

For four hundred years the thaller or dollar was a popular coin who's value was trusted all over Europe and the known (western) world.

When nation states became established, they created their own coins and would name them similarly, the American dollar, the Canadian dollar, etc.