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 degree scale.
Another one of those must haves in your photo kit. This is the standard scale that many Police Departments use.