When Weaver originaly invented the rail, they put cross-slots wherever they wanted them, with no regard to spacing.  The slots also varied a bit in width.  That meant that a device that worked on one Weaver rail often would not fit another rail.

The 1913 Picatinny standard fixed this, by standardizing all rail dimensions, slot widths, and slot spacing.

Seperate Weaver rings will fit either rail, assuming they only have one cross-bolt.  Some Weaver rails won't accept 1913 rings because the Weaver slots aren't wide enough.  Many 1913 mounts (i.e., devices with two or more cross-bolts, especially with fixed spacing) will not fit on a Weaver rail.

-Troy