The sofas are made of taped and plywood frames with iron wave springs and load-bearing iron supports. A really good quality sofa is still made with this traditional technology. This combination ensures decades of comfort and durability. The wave springs (flat steel strands stretched between the wooden frame under the seat) not only provide the flexibility and support that is important for the spine, but also distribute the greatest load on the seat. In this way, the fabric and padding do not start to sag or stretch at the points of greatest stress. It's an expensive technology, but furniture made this way will stay comfortable and looking new for decades, and a sofa made this way will last a lifetime.