tang sights often had open sights as well for close range use. Tang sights often had vernier scales, allowing adjustment down to a single minute of arc over the full range of the sight. Flip up sights Assault rifles and sporterized semi-automatic rifles can have foldable rear and front sight elements that can be readily flipped up or down by the user. Such iron sights are often used as secondary sighting systems in case the main weapon sight (typically an optical sight such as a telescopic sight or red dot sight) malfunctions or becomes unsuitable for the tactical situation at hand, and are therefore as backup iron sights (BUIS). Backup sights are usually mounted via Rail Integration Systems (most often Picatinny rails) in tandem with optical aiming devices, although "offset" BUISs that are mounted obliquely from the bore axis also exist. When used with non-magnifying optics (e.g. reflex or holographic sights), the flip-up rear and front elements often are designed to appear in the same sight picture, known as cowitnessing, as the primary optical sights. Adjustment Fixed sights are sights that are not adjustable. For instance, on many revolvers, the rear sight consists of a fixed sight that is a groove milled into the top of the gun's receiver. Adjustable sights are designed to be adjustable for different ranges, for the effect of wind, or to compensate for varying cartridge bullet weights or propellant loadings, which alter the round's velocity and external ballistics and thus its trajectory and point of impact. Sight adjustments are orthogonal, so the windage can be adjusted without impacting the elevation, and vice versa. If the firearm is held canted instead of level when fired, the adjustments are no longer orthogonal, so it is essential to keep the firearm level for best accuracy. The downside to adjustable sights is the inherent fragility of the moving parts. A fixed sight is a solid piece of metal, usually steel, and if firmly attached to the gun, little is going to be able to damage it beyond usefulness. Adjustable sights, on the other hand, are bulkier, and have parts that must move relative to the gun. Solid impact on an adjustable sight will usually knock it out of adjustment, if not knock it right off the gun. Because of this, guns for self defense or military use either have fixed sights, or sights with "wings" on the sides for