multi-function couplers. In the case of buffers and chain couplers, the draw gear behind the hooks, if any, will absorb the tension, while the side buffers will absorb the compression. Some couplers may not have a draw gear. Model railway couplers On model railroads couplers vary according to scale, and have evolved over many years. Early model trains were coupled using various hook-and-loop arrangements, which were frequently asymmetrical, requiring all cars to be pointing in the same direction. In the larger scales, working scale or near-scale models of Janney couplers were quite common, but proved impractical in HO and smaller scales. For many years, the "X2F" or "Horn-Hook" coupler was quite common in HO scale, as it could be produced as a single piece of moulded plastic. Similarly, for many years, a "lift-hook" coupler known as the Rapido and developed by Arnold, a German manufacturer of N-scale model trains, was commonly used in that scale. The chief competitor of both these couplers, more popular among serious modellers, was the Magne-Matic, a magnetically released knuckle coupler developed by Keith and Dale Edwards, and manufactured by Kadee, a company they started. While they closely resemble miniature Janney couplers, they are somewhat different mechanically, with the knuckle pivoting from the center of the coupler head, rather than from the side. A steel pin, designed to resemble an air brake hose, allows the couplers to be released magnetically; the design of the coupler head prevents this from happening unless the train is stopped or reversed with a mated pair of couplers directly over an uncoupling magnet. An earlier, mechanically tripped version of the design had a straight pin extending down from the knuckle itself, which engaged a diamond-shaped mechanical "ramp" between the rails, which had to be raised above rail height when uncoupling was desired. Once the Kadee patents ran out, a number of other manufacturers began to manufacture similar (and compatible) magnetic knuckle couplers. Recently, an exact-scale HO model of the AAR coupler has been designed and manufactured by Frank Sergent. This design uses a tiny stainless steel ball to lock the knuckle closed. Uncoupling is achieved by holding a magnetic wand over the coupler pair to draw the balls out of the locking pockets. In O scale, an exact-scale working miniature version of the "Alliance" coupler was manufactured from the 1980s by GAGO models in Australia. Since 2002 it has been marketed