Iron sights are a system of physical alignment markers (usually made of metallic material) used as a sighting device to assist the accurate aiming of ranged weapons (such as a firearm, airgun, crossbow or even compound bow), or less commonly as a primitive finder sight for optical telescopes. The earliest sighting device, it relies completely on the viewer's naked eye (mostly under ambient lighting), and is distinctly different to optical sights such as telescopic sights, reflector (reflex) sights, holographic sights and laser sights, which make use of optical manipulation and/or active illumination, as well as the newer optoelectronics, which use digital imaging and even incorporate augmented reality. Iron sights are typically composed of two components mounted perpendicularly above the weapon's bore axis: a rear sight nearer (or proximally) to the shooter's eye, and a front sight farther forward (or distally) near the muzzle. During aiming, the shooter aligns his/her line of sight past a gap at the rear sight's center towards the top edge of the front sight (which is usually shaped as a small post, bead, ramp, or occasionally, a ring), forming a line of aim that points straight at the desired target. Open sights are iron sights whose rear sight uses a notch of some sort, while aperture sights use some form of a circular hole. Most civilian, hunting and police long guns and nearly all handguns feature open sights, while many military battle rifles usually employ aperture sights. The earliest and simplest iron sights were fixed and could not be easily readjusted. Many modern iron sights are designed to be adjustable for sighting in firearms by adjusting the sights for elevation or windage. On many firearms it is the rear sight that is adjustable. For precision applications such as varmint hunting or sniping, the iron sights are usually replaced by a telescopic sight. Iron sights may still be fitted alongside other sighting devices (or in the case of some models of optics, incorporated integrally) for back-up usage. Principles In the case of firearms, where the projectile follows a curved ballistic trajectory below the bore axis, the only way to ensure it will hit an intended target is by aiming at the precise point on the trajectory at that target's intended distance. To do that, the shooter aligns his line of sight with the front and rear sights, forming a consistent line of aim (known as the sight