McGrath - there is an obvious difference between the tone of a string ringing its full length from the nut and one that's stopped some way up the neck with a capo. You don't need to be a guitarist to hear it. A capo is effectively a mute.

Don Firth - got an **actual example** of an early song collector making the mode-correcting mistake you describe? I can't think of one and I think you're fantasizing. (People trying to notate with microtonal accuracy, as you need for the music of India or the Islamic world, is a different matter).