In the case of firearms, where the projectile follows a ballistic trajectory, front and rear sights must be aligned with the line of sight of the shooter to the target, producing what is known as the "point of aim" (POA) within the shooter's field of view. Sighting in is a process in which the line of aim (known as the "sight axis") is adjusted to intersect with the trajectory of the bullet at the distance of the target, so the point of aim is calibrated to superimpose with the bullet's "point of impact" (POI). Iron sights provide horizontal and vertical reference points that allow the shooter to train the weapon on a target accurately.