Developed and patented by John O. Almen, the strip was originally supported by 2 knife edges; later improvements see it being supported on 4 small balls. The strip is placed in the chamber in place of the item to be shot peened, usually near to an area of the item where the result is deemed critical, sometimes located by a special fixture. Compressive stress introduced by the peening operation causes the strip to deform into an arch, which is measured using a gauge.