Because of its low profile and single tension construction, the derailleur does not hit the chainstay in rough riding conditions. The result is smooth and silent performance.
Various derailleur systems were designed and built as far back as the late 1800s. A French bicycle tourist, writer and cycling promoter Paul De Vivie (1853-1930), invented a two speed rear derailleur in 1905 which he used on forays into the Alps, and we wonder if 20 gears are enough..... Since then there have been many advancements, in the number of gears, materials and reductions in weight. Wiggle lists a large selection of front and rear derailleurs catering for double and triple systems with 7, 8, 9, 10 and even 11 speed cassettes.