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
Word sort words. Spelling or word study notebooks may be used to record, collect, and organize information about words and spelling patterns learned from the word sort. To complement the many 'hands on' activities students use for sorting words, digitized word sorts provide an efficient way for teachers to deliver spelling pattern differentiation. A variety of these are ready-made, including wordsortwizard.com. Research has shown that students who experience significant difficulty with spelling follow the same developmental course as other students, but do so at a slower pace. In such cases, it is critical to provide word sort words at the appropriate developmental