All factory Glock rear sights are interchangeable and elevation changes are made by switching out the rear sight (all factory Glock front sights are the same height). A shorter rear sight will lower the point of impact, whereas a taller rear sight will raise the point of impact. A one-step change (up or down) in rear sight height on a Glock 17 will result in approximately 2.4 inches of impact difference at 27 yards.Glock rear sights are identified by markings on the right side consisting of one or more horizontal bars. A single bar indicates the most common rear sight height: 6.5mm (.256 in). A long bar with a short bar beneath it indicates a 6.1mm (.240 in) height and a long bar with a short bar above will measure 6.9mm (.271 in) high. The tallest of the factory Glock rear sights is the 7.3mm (.287 in) with markings consisting of a long bar with two shorter bars above the long bar.