body mounted in jig to mortise neck joint. the lower bout face is the reference plane, and the top side of the upper bout where the mortise will be cut is off of square to that plane by a few degrees - thus the body does not sit flat on the horizontal plate. tape applied to the centerline of the soundboard and used as squaring reference when clamping the body to the jig. squaring is further helped by seeing the tape/center line through the mortising slot.