Made from 1/8" acrylic. Use for the top and back radius

Has both concave on one side and convex on their respective sides.

Useful in checking the radius for the top and back. 21"  Long enough for the length of the guitar.

Also useful for marking brace material for pre-shaping the guitar radius. 

You can use a router to copy the radius onto braces, or trace with a pencil and reach the desired radius with hand tools.