Single-speed bicycle builders have devised a variety of methods for producing and maintaining the proper chain tension. The most common method is horizontal rear dropouts or track ends. These allow the rear wheel to be moved fore or aft as necessary. Using chain tugs with track ends ensures that optimum chain tension can be achieved with the wheel remaining centered. A similar design, sliding dropouts, uses detachable dropouts that are bolted to the frame through horizontal slots. This allows both quick release or bolt on hubs to be used, or through-axle hubs with the appropriate dropouts. Sliding dropouts usually have set screws that bear on the front of them through the frame to adjust their position for tensioning and wheel alignment. The main advantage of sliding dropouts over track ends is realized when they are used with disc brakes, as the brake caliper mount is part of the left dropout, and thus moves with the dropout, maintaining the hub to caliper distance. Sliding dropouts can also include a derailleur hanger if multi-speed use is desired.