Made to rigid FBI specifications, the "Bureau" scale set contains a 15cm x 30cm L-Shaped scale and a 15cm straight scale. The scale set provides maximum contrast by featuring black markings on a white background on one side and white markings on a black background on the reverse. Alternating black and white bands provide an excellent visual reference from a distance and crosshairs at each end provide a means for checking and correcting perspective distortion. The L-shaped scale is rigid and will always provide an exact 90° scale.