I'm guessing it's probably because the nut height is taken out of the equation that way. Obviously you can't compare 12th fret action heights measured on fretted strings with ones measured on open strings.James Lister wrote:Does he say why? I can't think of a reason. Given that most people measure with the strings open, it can only lead to confusion.David_Norton wrote:I seem to recall David Schramm saying that the fretted method was the best way. May have been on the old AG Forum though.
But, for example, when two guitars measure 3.0mm on the high e string (measured on open strings), if there is any difference between the nut heights of the guitars, which is possible, then the 12th fret action heights on the guitars are actually not as identical as the measurements indicate. Actually, the bass strings are where you'll see the most variation in nut height among different well made guitars. It can easily be enough to affect the 12th fret measurement.