Road rear derailleurs: when speed & accuracy are key
The rear derailleur is responsible for changing the gear from one cassette sprocket to the next at the rear wheel. Road bike rear derailleurs must therefore be extremely accurate, so you're always in the gear you want to be, and quick! That means no slow changes or skipped gears. When you buy a rear derailleur, make sure it's compatible with the number of gears and size of your rear cassette. For bigger cassettes, you'll want a medium or long-cage rear derailleur that can comfortably get the chain onto the biggest sprocket. Gravel bikes also usually need medium or long-cage rear derailleurs. It's also not a good idea to mix and match drivetrain components from different brands/groupsets.