Picatinny Rail

The Picatinny rail (/ˈpɪkətɪni/ or /ˌpɪkəˈtɪni/), also known as a MIL-STD-1913 rail, or Standardization Agreement 2324 rail, is a bracket on some firearms that provides a standard mounting platform consisting of rails with multiple transverse slots similar in concept to the earlier commercial Weaver rail mount used to mount telescopic sights.

Whereas the earlier Weaver design is based on a low wide dovetail, the Picatinny variation has a more pronounced angular section. Designed to mount heavy sights of various kinds, a great variety of accessories and attachments are now available and the rails are no longer confined to the rear upper surface (receiver) of long arms but are either fitted to or machine milled into the upper, side or lower surfaces of all manner of weapons from crossbows to pistols and long arms up to and including anti-materiel rifles.

Accessories – which can include vertical forward 'tool' or 'pistol' grips; bipods and rests; electro-optical sights, including image intensifiers; flashlights and laser sights – can be mounted by sliding into place (or, if already fitted with a Weaver mount, clamped to the rail), thus providing backward (one-way) compatibility with any items already provided with Weaver-type mounts.

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Wikipedia