Bolts are headed, externally threaded fasteners used to keep two objects, often metal or wood, in a fixed position relative to one another. Unlike screws, which cut their own threads, bolts pair with the internal threads of nuts to stay in place and ensure the desired level of tension in the bolted joint. The use of nuts aids in disassembly and reassembly. In addition, bolts generally have a greater load bearing capacity than screws but lack a tapered shaft. The head style of the bolt determines the tool required for tightening and loosening. Bolts are responsible for holding together everything from furniture to machinery to bridges.