A coil spring with a cone both top and bottom. In seats these are traditionally sewn and tied over Herringbone (Black & White) webbing. Do not forget that new springs will be very strong in comparison to those you have removed. It may be necessary to replace with a size smaller to be kind to an old frame. Also think carefully if replacing only some springs in a seat, it may be better to replace the whole set. The higher the gauge number the thinner the wire.