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 articles and determiners French indefinite article is analogous to the English indefinite article "a/an". Like "a/an", the French indefinite article is used with a noun referring to a non-specific item, or to a specific item when the speaker and audience do not both know what the item is; so, « J'ai cassé une chaise rouge » ("I broke a red chair"). Unlike "a/an", the French indefinite article has a plural form, often translated as "some" but usually simply omitted in English; so, « Il y a des livres là-bas » ("There are some books over there" or "There are books over there"). The