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
French corsairs of the French Crown, if captured by the enemy, they could in principle claim treatment as prisoners of war, instead of being considered pirates. Because corsairs gained a swashbuckling reputation, the word "corsair" is also used generically as a more romantic or flamboyant way of referring to privateers, or even to pirates. The Barbary pirates of North Africa as well as the Ottoman Empire were sometimes called "Turkish corsairs". The word "corsair" comes directly from the French word "corsaire", itself borrowed from the Italian "corsaro". This derives from the Latin "cursus", meaning "course" (as in journey or expedition). The French