This is a Topeak rear carrier compatible with disc brakes. Originally, it was designed for the BabySeat II, which has rails for attaching a child seat, but I used it as a regular carrier. You can attach things like a rear basket to the rails, and it's easy to remove, making it very convenient. I bought it four years ago, but I've taken it off, so I'm selling it cheap. There are many scratches, but that's to be expected with use, so please only consider buying if you don't mind them. It's for 29er and 27.5-inch wheels. Please note that this is just the carrier itself, and does not include mounting hardware. However, you can purchase the hardware separately on Amazon, etc. No returns. I'm offering it at a bargain price as a result.