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
Old English subjunctive Old English subjunctive The subjunctive mood is a flexible grammatical instrument for expressing different gradients in thought when referring to events that are not stated as fact. It is still used frequently in such languages as French, German and Spanish, and also in languages outside the Indo-European branch such as Turkish and Hungarian. In modern English only remnants of a once complex system of separate conjugations exist. What once could be expressed succinctly with the mere change of a conjugation is now only expressible, more often than not, by using word-laden modal constructions. The word subjunctive as used to denote