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
Influence of French on English Several elements must be observed. The following French glossary in English is in no way exhaustive. These words come as examples to illustrate the countless French words that are part of the English language. In this section, examples of French-to-English lexical contributions are classified by field and in chronological order. The periods during which these words were used in the English language are specified as much as possible. It is not always possible to state with certainty the precise period from which a word was borrowed or integrated. The English word is on the left, with its current French equivalent