Weaver

The Weaver system is a cross-slot mounting standard that predates and inspired the modern military pattern. Its scope-base uses transverse slots cut into the top surface, into which the recoil lug of a ring or accessory drops to keep it from sliding under recoil. The slots are relatively narrow, and crucially their spacing is not standardized, so the positions vary from one base to the next.

Those irregular slots are the key difference from the picatinny-rail, whose wider slots sit at a fixed, repeatable interval that allows precise and interchangeable positioning. As a rough rule, Weaver-spec scope-rings often fit a Picatinny rail, but Picatinny rings will not reliably fit the narrower Weaver slots. While largely superseded for serious work, the Weaver pattern, along with the simpler dovetail, still appears on rimfires and budget optics.