This is a 50 tooth chainring with attached chainguard for Brompton bikes. You may need to replace a damaged or worn down 50 tooth chainring you already have, or you may be looking to reduce your gearing if you have a single speed or a 2-speed that came with a 54 tooth chainring. If your case is the latter, keep in mind you may need a different length chain; be sure to check out our stock of Brompton chains to find the right one.