Regular steel 3/8" imperial UNC threaded bolts. Suited for indoor climbing walls. Will corrode if exposed to water and high humidity. There are two types of bolts: Regular Socket Head Bolts and Countersunk Heads. The only set of holds that we sell that requires the Countersunk Heads is the Classic Footers Set, all other holds require Regular Socket Head Bolts. Please be sure you order the correct type. If you select bolts when you purchase holds, the appropriate bolts will be supplied.