Is your front shift controller an indexed one? Is it a mountain bike or road bike one (as well as the crankset type)? Ft ders have a few specs that should all coordinate to insure best performance. Already mentioned is the direction of cable pull, bottom or top. But also mentioned is that some Shimano ders have both capacities. The next design aspect is whether the parallelogram is above or below the cage as in bottom swing or top swing. This is more about the interference with frame designs like suspension pivots, seat tubes that get non round at the shell or bottle bosses located where the der clamp might be. The there's the obvious aspect on mounting interface. What tube diameter for a clamp around one (and again some ders have shims for the common tube sizes), braze on mounts, E bracket and now direct mounts are the usual versions issues. A factor not often mentioned is that some ft ders have different cage movement for the same cable pull movement. Road and mountain ft ders can be different in their cable pulling needs as well as the cranks can have different ring spacing (center to center) between road and mountain cranks. There's the cage shape. Double rings usually have an inner cage plate with less height then triple ders have. Some cages are curved along their lower edges to better follow smaller rings. Lastly there's the cage width. Wider chains or a lack of ft shift controller trim functions mean wider or more sculpted cages. Andy