And after the throne of Alinor did finally break at the feet of Men, and news of it came to the Dragon Emperor in Cyrodiil, he gathered his treasurers and spoke to them, saying:

"'You have suffered for me to win this throne, and I see how you hate exchange rates. Let me show you the power of Talos Stormcrown, born of the North, where my breath is long spreadsheet. I breathe now, in royalty, and reshape this economy which is mine. I do this for you, Red Treasury, for I love you.'"

And thus, all gold coins everywhere became Septim Gold Coins, the standard currency of the Third Tamrielic Empire, from Alinor to Port Telvannis. Even those buried long forgotten in old Nordic barrows, Ayleid ruins, and Dwemer ruins.

LOL. Well, that's one way to explain that, I suppose.

Now I understand!
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It would have been nice if the TES games had multiple textures for coins