Last week I mentioned that the Romans had a base 12 (“duodecimal”) system for fractions. Crazy! But there was a method to their madness. Multiplication and division are easier in base 12 than in base 10. 12 has a bazillion factors (1, 2, 3, 4, and 6). We still see remnants of the Romans’ duodecimal system in some of our measurement systems and especially in our 12-hour, 60-minute clock.
As it happens, while base 10 is pretty much universal in the modern world, in the ancient world it was anything but.
The Mayans used base 20. So did the ancient Celts and the Maori.
The ancient Egyptians in the Old Kingdom used a binary numeral system. THINK WHAT AMAZING COMPUTER PROGRAMMERS THEY WOULD HAVE BEEN.
And base 5 is found in many ancient cultures, often with the word for “5” being the same as the word for “hand” or “fist.”
It is seldom necessary in a fantasy novel to world-build to the level of how the characters write their numbers but if you do, you have the freedom to get plenty creative. (There is even a tribe in Papua New Guinea with a numeral system that’s base 27.) Where you’re most likely to need something numerical is in dealing with money. Whatever your world’s currency is, how is it split into smaller coins? Fourths, fifths? Tenths? Twelfths? Halves? And what are the smaller coins called?