axis) and in turn producing what is known as the point of aim (POA) within his own field of view, which then gets pointed directly (i.e. aimed) at the target. The physical distance between the front and rear sights is known as the sight radius, the longer of which produces smaller angular errors when aiming. "Sighting in" is a process in which the sight axis is adjusted to intersect the trajectory of the bullet at a designated distance (typically at 100 yards/meters), in order to produce a pre-determined point of impact (POI) at that distance, known as a "zero". Using that "zero" as a default reference, the point of aim can be readily re-calibrated to superimpose with the bullet's point of impact when shooting at different distances. Modern iron sights can all provide some horizontal and vertical adjustments for sighting-in, and often have elevation markings that allow the shooter to quickly compensate (though with rather limited precision) for increasing bullet drops at extended distances. Because the sight axis (which is a straight line) and the projectile trajectory (which is a parabolic curve) must be within the same vertical plane to have any chance of intersecting, it will be very difficult to shoot accurately if the sights are not perpendicularly above the gun barrel (a situation known as canting) when aiming or sighting-in. Rear sights on long guns (such as rifles) are usually mounted on a dovetail slot on the back part of the barrel or the receiver, closer to the eye of the shooter, allowing for easy visual pick-up of the notch. Front sights are mounted to the front end of the barrel by dovetailing, soldering, screwing or staking very close to the muzzle, frequently on a "ramp". Some front sight assemblies include a detachable hood intended to reduce glare, and if the hood is circular, then this provides a reference where the eye will naturally align one within the other. In the case of handguns, the rear sight will be mounted on the frame (for revolvers and derringers) or on the slide (for nearly all semi-automatic pistols). Exceptions are possible depending on the type of handgun, e.g. the rear sight on a snub-nose revolver is typically a trench milled into the top strap of the frame, and the front sight is the to-be-expected blade. Certain handguns may have the rear sight mounted on a hoop-like bracket that straddles the