I understand there are fret calculators to figure fret positions. I have used them to draw and cut different lengths of banjo fretboards. The old banjo scales have become an interest to me. In my Internet search I understand the luthiers of old used the "18" method of figuring their fret spacing for a particular scale. I know the "18" rule this has been more refined to "17.817 rule"!
My question is this: Do we know exactly what Gibson used in the 20's and 30's to figure their fret scales? I understand they used a "stretch" scale. This allowed the upper resister of frets to be slightly closer together to allow for the distance the strings were off the frets. This allowed for the "stretch" of the string to the fret. I have done the math and created a scale using the "18" rule. In comparing it to the scale figured on the 17.817 formula. There's not much difference in the two. The "18" rule scale does back up the upper register of frets! This allowing for string stretch and a truer noting fretboard, so i have been told. This leads me to believe that Gibson may have used something different than the 17.817! Even the scale of my '33 tenor neck seems to be "compressed" a little compared to the computer generated scales using the 17.817 formula of the same scale.
Maybe Joe Spann or some of the Gibson historians could weigh in on this and shed some light on it for me.