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 will for most students lead to the abstraction of spelling patterns and the sounds to which they correspond. Word sorts can be teacher directed (closed) or student directed (open). For example, students in the with-in-word pattern phase of word knowledge could sort words according to a vowel pattern; in such sorts there is always a miscellaneous category for words that do not follow the target categories. Game-like formats such as board games and card games can also be effective if they focus on words that reflect spelling patterns. Word-building activities also facilitate abstraction of pattern: word wheels, flip charts, making