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 has 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. There are many fitments of front derailleur and if unsure it is worth viewing Wikipedia which has a great page dedicated to front and rear derailleurs and could help with any fitment or compatibility queries you may have.