system
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human
ELI5: Why does musical theory define both sharps and flats? Couldn't we just define them all as sharps? Why do we need both?

Thanks, chaps! So in summary.

* To keep one of each every letter in any key.
* Sharps used to be legitimately different from flats.

Thanks for E-ing L I was 5.
gpt
I believe it's just so that every key uses all 7 letters with no duplicates

For instance, consider the key of F Minor: 
F, G, A♭, B♭, C, D♭, E♭

If we only allowed sharps, we'd have to convert it to
F, G, G#, A#, C, C#, D#

It's just a bit easier to keep things straight when every key moves through the alphabet the same way, and using both flats and sharps just gives some naming flexibility that allows that to happen.