... an 'under pillow case' (usually quite thick, coarser cotton) and an upper (matching the bed linen or just plain white, but fine thread count)?
I don't think I am, but my cleaner seems utterly unable to understand, and she pulls off both cases on some pillows, and replaces with the coarse ones on top. I always spend ages going through sorting all the beds out, rewashing etc. Even though I've tried to explain it to her lots of times. I think I need diagrams.