Sight height is the distance, in inches (or cm), that the sights sit above the center line of the barrel's bore. With telescopic sights, this distance is usually 1.5 inches. With iron sights, this distance is usually 0.9 inch. Sights mounted below the barrel have a negative height. For example, some laser sights are mounted 2 inches below the barrel, and thus, have a sight height of -2.0 inches.