The back of this guitar has the shape of a bowl with a 15 foot radius. The top is flat. The rim of the guitar with its feminine profile has to match the shape of the bowl. To do this I use a large sanding plate with the proper bowl shape hollowed out of the plate. I will run the guitar rim over the sanding plate until I get a good match between the two. I mark the edge of the rim with a magic marker and when I have sanded away all of the marker I know the match is good.