I was a Springer Field Trial gun for years - looked at the rump of a lot of phez! As a fly tier, however, the "Cathederal Window" rump feather is more interesting. Attached to the underside of that feather is a thing called a phylo plume, which is a great feather for imitating gills on large mayfly nymphs like the "Hex" The left feather is the phylo plume, and the right is the Cathederal Window feather