Zig Zag springs: Also known as sinuous, serpentine or no-sag springs are easy to install and very comfortable. The pitch is measured by the distance between the centre points of the 'U'. Made from tempered wire, supplied flat. The measurement is the size of the frame.
These are installed onto a frame with spring clips (sold separately). The spring is usually longer so it has a slight crown/arch. Tie the springs together in two or three spaces with jute twine, then cover the springs with hessian before adding the foam.