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Crosshairs
The simplest type of reticle is called a "crosshair" --- two lines that intersect each other in the middle of the scope's field of view. Originally, the need for thin, strong materials saw crosshairs made by intersecting two literal pieces of hair; today the image is more often created by two crossed pieces of wire. Crosshairs are extremely simple to make, but they require training to use effectively --- you have to look exactly down the center of the scope and at a particular distance from the eyepiece (the "eye relief" of the scope).
Range-finding Reticles
Plain crosshairs must be precisely adjusted to give an accurate shot, and many rifles intended for use at longer ranges use a more complex system that must be etched onto a piece of glass embedded in the scope. These range-finding reticles use small printed brackets, directly overlaid onto the image of the crosshairs, to help the shooter find his distance to the target using a stadiametric method --- a target of known size is viewed through the scope and positioned against precisely measured lines in the scope's optics to determine how far away it is.
Reflex Sights
Physical reticles in scopes suffer from the same problem: as your eye moves, the crosshairs appear to point in different directions as a result of parallax shift --- to see this in action, hold up a pencil in front of your face and watch the object located behind the pencil's tip. As your head moves even slightly back and forth, the tip appears to point at a different object. Reflex sights, or reflector sights, partially get around this problem by projecting an image (frequently a small red dot --- reflex sights are sometimes known as "red dot" reticles for this reason) at an infinite focal distance, rather than one located within the scope.
Holographic Sights
Holographic sights operate similarly to reflex sights in that they show an illuminated image superimposed over a piece of glass in the scope. However, they use a three-dimensional image known as a "hologram" that is projected against the glass of the scope by means of a laser. Holographic reticles frequently take the same form, such as small dots or tiny crosshairs, as those used in reflex sights. Reflex sights require a partially reflective mirror that can limit the amount of light that gets through a scope; holographic sights do not have this problem, although because they use lasers instead of LEDs their power consumption is much, much greater and many holographic sights turn off automatically after several hours of use.