sh_ and ch_ compose the same words

[MarcoP]

Marco, that's fascinating, thank you. So what you're saying is that sh vs ch are interchangeable with a few very defined exceptions?

Yes, as nablator wrote above, the two benches are mutually replaceable (in particular as prefixes). I guess this could also be the reason for davidsch's OP; Torsten's numbers point in this direction as well. I am not sure that the exceptions are very defined, but shedy and sho have a considerable impact because they are so frequent: maybe they could explain the difference in average length between ch-words and sh-words.