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 certain elements, young children make sense of words and patterns within words. Word sorts combine both constructivist learning and teacher-directed instruction. Students receive a stack of cards containing either pictures or words that have several types of contrasting sounds, patterns, or meanings. Each student must then figure out the patterns that exist within the stack, and sort the cards accordingly. By doing this, students construct their own knowledge of words, creating a lasting understanding of how language works. This process contrasts greatly to the simple memorization related to traditional spelling tests. The more students understand about the structure of words