The Picatinny rail (/ˈpɪkətɪni/ or /ˌpɪkəˈtɪni/), also known as a MIL-STD-1913 rail, STANAG 2324 rail, or tactical rail, is a bracket used on some firearms in order to provide a standardized mounting platform for accessories and attachments, similar to the Weaver rail mount. The rail consists of a series of ridges with a T-shaped cross-section interspersed with flat "spacing slots." Accessories are mounted either by sliding them on from one end or the other; by means of a Weaver mount which is clamped to the rail with bolts, thumbscrews or levers; or onto the slots between the raised sections.
The rail was originally for scopes. However, once established, the use of the system was expanded to other accessories, such as tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets. Because they were originally designed and used for telescopic sights, the rails were first used only on the receivers of larger caliber rifles. But their use has extended to the point that Picatinny rails and accessories have replaced iron sights in the design of many firearms, and they are also incorporated into the undersides of semi-automatic pistol frames and even on grips.