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 understanding that can be developed in a variety of ways. Comparing sounds is the easiest task for developing phonemic awareness. Sound sorts can be introduced very early on and develop strategically throughout primary learning. An ability to sort sounds is essential for early reading because sound manipulation is a stepping stone to word study and decoding ability. For some students a direct instructional approach is not necessary in the development of sound study and sorting. Other children who have had less exposure or lack the understanding of sounds and their manipulability may need further instruction to develop their ability. Teachers