Derailleur gears several standards in use, and in each the product of the derailleur's actuation ratio and the length of cable pulled must equal the pitch of the rear sprockets. The following standards exist. Some rear derailleurs, especially for mountain bikes, incorporate a clutch to keep the lower length of chain in sufficient tension to prevent the chain from striking the bottom of the chain stay, called "chain slap", and which can damage the chain stay. Clutches are also helpful in preventing the chain from derailing from the chain ring on systems without a front derailleur. The front derailleur only has to
Units of measurement in France before the French Revolution (league) had a number of variations, particularly the "aune" (which was used to measure cloth The "loi du 19 frimaire an VIII" (Law of 10 December 1799) states that one decimal metre is exactly 443.296 French lines, or "3 pieds 11.296 lignes de la "Toise du Pérou"". Thus the French royal foot is exactly 9000/27,706 metres (about 0.3248 m). In Quebec, the surveys in French units were converted using the relationship 1 "pied" (of the French variety, the same word being used for English feet as well) = 12.789 English inches. This makes the Quebec "pied" very slightly smaller (about