If you mix a square wave of one duty cycle with another of a different duty cycle, they partially cancel each other out and you get a new sound. I'm not sure,

If you mix a square wave of one duty cycle with another of a different duty cycle, they partially cancel each other out and you get a new sound. I'm not sure, but I believe the original NES Castlevania does this in some places, like in the "you died" jingle. (It's possible I'm misremembering and it's simply two square notes separated by an octave.)