Iron sights are a system of shaped alignment markers (usually metal) used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector (reflex) sights. The earliest and simplest iron sights are fixed and cannot be easily adjusted. Many iron sights are designed to be adjustable, so that the sights can be adjusted for elevation or windage. On many firearms it is the rear sight that is adjustable.