A compression spring is an open-coil helical spring that offers resistance to a compressive force applied axially. Compression springs are usually coiled as a constant diameter cylinder. Other common forms of compression springs--such as concave, convex, conical, tapered or various combinations of these, are used as required by the application. While rectangular  / square, or special-section wire may have to be specified, round wire is predominant in compression springs because it is readily available and adaptable to standard coiler tooling.