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 the English definite article "the", although they are sometimes omitted in English. The French definite article can vary according to the gender (feminine or masculine) and number (singular or plural) of the noun. The definite article takes the following forms: Like "the", the French definite article is used with a noun referring to a specific item when both the speaker and the audience know what the item is. It is necessary in the following cases: Unlike "the", the French definite article is also used with mass nouns and plural nouns with generic interpretation, and with abstract nouns. For example: The