The measure of the curvature of the top of the fretboard from edge to edge is often incorrectly referred to as the "neck radius". Actually, the correct terminology would be either "fretboard" or "fingerboard radius" and the actual neck shape and size should be called "neck profile". The fretboard radius can be found by first drawing a circle with a corresponding radius, (the "radius" is the distance from the center of a circle to its outer edge), and then cutting out a portion of that circle corresponding to the width of the fretboard. For example, if you have a 7 1/4" radius fretboard. You could tie a piece of string to a pencil, measure out a length of string to 7 1/4", and put a thumbtack on the other end of the string. Secure the tack, stretch the string, and draw a circle. By cutting out a piece of that circle the width of your fretboard, you will have an example of an arc with the same curvature as that of your fretboard radius.