How do you measure the span between the thumb and the pinky?
It is inacurate when keeping the hand up.
If I place it palm down on a sheet of paper and mark with a pencil, it seems I am getting a little bit more than attempting to measure directly the hand.
Anyway, measured on a piece of paper:
- when spread but still quite relaxed (I can keep it that way as long as I want) : 23 cm = 9 and 1/16 inches or so.
- when spread as far as it goes until it almost hurts (without the help of the other hand or the surface on which the paper sheet is placed, of course): 24.5 cm = 9 and 10/16 inches
I manage to play with 1st and 4th fingers on frets 1 and 6 on any two strings (but prefer not to - it requires a very forward-down-right elbow position),
or barre on 1st fret and 4th finger on 5th fret - not very easy, but quite doable.
On piano, I can play all the 10ths, although the worst of them C#-F - just barely, also G#-C is quite difficult,
I can hit the all white ones not only with the thumb from the side, but also from above, but with no room to spare, with quite spread out and tense hand.
So chromatic scales in tenths or longer fast walking tenth runs are painful - no time to relax the hand in between the notes.
I can also play octaves on piano with piano fingers 2-5 = guitar fingers 1-4
But all the above is with the left hand only, the right hand cannot play tenths or octaves with 2-5 fingers, that is because the span of the left hand increased gradually with stretching and I did no stretching of the right hand.
Perhaps piano is a good indicator do I need 640 mm scale or not?
If my calculations are right, the 640 mm scale reduces the stretch between frets 1-6 by 2.37 mm and between 1-5 by 1.95 mm,
but at the higher positions there is less room left for fingers by:
on 9th fret - 0.35 mm
on 12th fret - 0.3 mm
on 15th fret - 0.25 mm
on 19th fret - 0.2 mm - I don't think I ever played on 19th, though.
So 2 mm gain is probably more noticeable than 0.3 mm loss, therefore 640mm scale perhaps is a good solution for those who need it.
But do I need it?
Until now I was always playing 650 mm, never occured to me that I might need 640 until I saw here this 9 inch rule
I am thinking about getting Pavan TP-30 spruce top, so I could choose TP-30-64 instead but I'd rather have the regular one if I don't really need 640.