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.
These are installed onto a frame with spring clips. The spring is usually longer than the width of your frame so it has a crown/arch. Tie the springs together in two or three spaces with jute twine, then cover the springs with hessian before adding the foam.