That test that Mark Clark described was part of the London college of music tests for kids way back in the 1920/30s. (No, I wasn't there!!!)

Playing bass, I discovered that the only tuning that made sense was to ask the guitarist to play a chord of A minor (not major) and I could tune the D string to that at the 5th, and then relative-tune the other strings. But I could never tune the bottom E on the bass by ear.

- Ian B