There are many things the crime scene technician has to know about scales and their proper use in order to take photos of good value to the forensic examiner. Choosing the correct scale and using it properly can make the difference between a positive identification and an inconclusive result. Scales are used to provide the viewer with an idea of the size of a mark or an impression. They also allow an examiner to enlarge the photo to its natural size for comparison purposes. Without a scale, it is difficult to compare the characteristics of such evidence as marks or impressions. The first step is choosing the right scale for the job. A good scale has a non-reflective, matte surface that does not create bright reflections in the photograph. It should provide some aid in correcting perspective. A scale with right angle or perspective circles allows for this correction. Scales should be thin and flat. A good scale is also rigid so that it does not bend or droop. The Federal Bureau of Investigation (FBI) developed a set of scales that meets all of these criteria, named the “Bureau Scale Set.” It consists of an L-shaped scale that is 30 cm long on the long arm and 15 cm long on the short arm, and another scale that is 15 cm long. The L-shaped scale is commonly used for shoe impressions, and is excellent for that application. The American Board of Forensic Odontology developed the “A.B.F.O. No. 2 scale,” which is designed for photographing bite marks but used for many types of evidence. Placing the scale on top of or in the impression may damage the impression when placing and/or removing the scale. When close-up photographs are taken without a scale, the scale may be placed beside the evidence. Using scales properly is easy and takes little additional time when photographing evidence at crime scenes. The result is better evidence, better laboratory examination results, and more convictions.