Very hard and uncomfortable foam.  The backrest does not taper off at the top with a soft edge as the photos would suggest. The top edge that looks like extra cushioning above the wood is in fact just another board wrapped in a very thin layer of foam and upholstery.  It digs in to your spine if you lean back.  Photo attached with me pressing my finger on the edge to show how thin the cushioning is.  Quality is also lacking with inconsistent stitching and geometry especially at the corners.