Boards from 1 to 4 in (2.5 to 10 cm) thick and of sufficient length to bridge the space between timber or steel sets or between rings in skeleton tubing. Usually, planks 9 to 12 in (23 to 30 cm) in width are used. Round poles, either whole or split, light steel rails, ribbed sheet metal, and reinforced concrete slabs are sometimes used in place of planks. Backing deals tighten the supports against the ground and also prevent the collapse of material between the timber or steel sets or rings.