The two main rail systems when installing a scope on your rifle are the Weaver and the Picatinny (MIL-STD-1913). Although very different, the two rail systems can be used interchangeably in most cases.
Weaver Rail System or Picatinny Rail System Comparison
What’s the Difference Between Weaver vs. Picatinny
The main difference between these two rail systems is the size of the slot spacing and cross slots. Weaver rails have a slot width of 0.180″ (4.572 mm) but are not necessarily consistent in the spacing of slot centers. The Picatinny rail has a slot width of 0.206″ (5.232 mm) and the spacing of slot centers is always 0.394″ (10.008 mm).
These measurements all Weaver devices to fit on Picatinny rails, but not all Picatinny devices will fit on Weaver rails.
Rule of Thumb: If your scope came with Weaver-style rings, it will most likely work on both rail systems. If the scope came with Picatinny-style rings, it will most likely only work on a Picatinny mount.
Weaver Rail System 101
Weaver Optics was founded in 1930, by William Ralph Weaver, which designed the Weaver rail mounting system for his scopes. The first Weaver rails were two-piece mounts, one attached in front of the receiver, the second attached to the rear for mounting the scope.
The two-piece mount allowed for bolt-action rifles to operate freely, and allow cartridges to be loaded quickly. It was also easy for stripper clip loading that was commonly used in the military.