The CR-A19 chain guide from XLC is designed to fit the space where your front derailleur would be if you are using a double chainring configuration. It is compatible only with seat posts that have a diameter of 31.8 mm or 34.9 mm. It is suitable only for monoblock drives, specifically for 1x9, 1x10, and 1x11 systems. Chain guides are typically found on mountain bikes as they prevent the chain from dropping off the chainring, which can happen in particularly rough terrain or during shifting. The CR-A19 is made from 6061-T6 aluminum, ensuring that it is both durable and lightweight. This item is available only in black but fits many bicycles.