16" risers: Standard and Extreme
14" risers: SXT,ZSR,and Nitro.
The SXT and ZSR have a slimmer riser profile.The limbs on these 2 bows are interchangeable. The same limbs will be 2 pounds heavier on an SXT then on a ZSR riser. Due to the slimmer profile, these limbs will also not be compatable with the 16" risers.
The limbs are interchangeable between the Standard and Extreme risers. The Extreme will also be 2 pounds heavier, then if they were on a Standard riser.
The Nitro is a completely different animal. The limbs will not work on any of the other Zipper risers.
Hope this helps !!! Please correct me if I screwed up anything Bill. :D