Recently, manufactured steel and aluminum have also been used. The wall may have places to attach belay ropes, but may also be used to practise lead climbing or bouldering. Each hole contains a specially formed t-nut to allow modular climbing holds to be screwed onto the wall. With manufactured steel or aluminum walls, an engineered industrial fastener is used to secure climbing holds. The face of the multiplex board climbing surface is covered with textured products including concrete and paint or polyurethane loaded with sand. In addition to the textured surface and hand holds, the wall may contain surface structures such as indentions (incuts) and protrusions (bulges), or take the form of an overhang, underhang or crack. Some grips are formed to mimic the conditions of outdoor rock, including some that are oversized and can have other grips bolted onto them.