Pulleys comprise a wheel (sheave) and are used in work at height to reduce friction in a rope or wire system. When used with other pulleys in hauling set-up, they will provide mechanical advantage. Pulleys differ in size, design and material. The larger the pulley, the less friction there is on the rope or wire passing over it. Available as a single wheel, as a twin (side by side wheel) or double in-line (design to be used on a tyrolienne). Ones made from stainless steel offers more durability, but this makes the pulley heavier than the more popular ones made from aluminium. Additional features may include the inclusion of progress capture system (normally a cam which lets the rope pass in one direction, but locks onto the rope in the other direction). A more traditional one-way locking system is to use a prussik (prusik / prussic) with some pulleys designed with extended side plates to stop the prussik being pulled over the pulley wheel. Some pulleys have a one way locking wheel, which helps provide extra friction when lowering.