They are done with webbing over the frame , not a flexible type usually . It gets tacked on the under side and woven under and over to the other side with roughly a two or three finger gap between each length. No , backsides don't feel them really , it's one of the trade standard ways of upholstering a seat. I'm no upholsterer at all but I've worked on plenty of them, repairing the woodwork or building them . Ill see what I have to take a photo of. Rob