 definition..
= The Diameter across if it were a complete circle.
Make a 15" circle. Take 3" of it and that is lets say your Fingerboard width. That Radius is 15" which the 3" wide Fingerboard is a section of the circle. If it's a 4-string or 7-string, it is still the slice of the same pie.
Tighter pies/circles/radius's require thicker fingerboards in the center. Shallower require less.
To maintain the same thickness at the edge where the side dots go, we make a larger Radius (shallower curve) as we add strings or make wider model fingerboards.
For the center to the edge of the side of the fingerboard we call this the drop off point. With the center being about 1/4" thick and the edge being about 3/16" thick, the Fingerboard must have a shallower radius (larger circle/pie) in order to maintain the same drop off point and fingerboard thickness overall from model to model.
For us these are estimate Radius numbers only for our Basses. Each Fingerboard is first rounded by a shaper cutter and then hand sanded and re-leveled on the neck just before fretting and after all the carving and sanding is done to ensure the desired spec. >> Estimates; 4=15", 5=18", 6=24" and wider 5 and 6 or 7s I can't recall. These numbers are in my head for the last 20 years. I don't know the exact finished radius but it gives you an idea. The drop off point is what we aim for. The Radius is what ever it is when we achieve the drop off ratio. The radius is also slightly different from end to end. The #s are only the custom cutter size we start with on the shaper. Hand work makes it into a playable musical instrument. | 
