Alumicolor Straightedges with Center Finding Backs are really two rulers in one. The front side is calibrated in inches and millimeters while the back side has center finding calibrations in inches. They have ultra smooth edges and photo anodized calibrations that are restistant to cleaners and will not rub off. Made of sturdy, rigid aluminum they are 1/16" thick by 1-1/8" wide. The 30", 36" and 48" lengths are 1/8" thick by 1-1/4" wide.