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The rear derailer is one of the main components whose improvement made the modern bicycle pleasant to use. If you've ridden a ten-speed from the 70s or earlier, you know what I mean. Shifting nowadays is buttery-smooth when everything is well set-up with good cables, and for that the derailer, cassette and chain had to be designed together to enable indexed shifting.
That said, derailers are now so good that you can fudge things like compatibility in surprising ways, and you'll benefit from the improvements even if you're using old-school friction shifters.
In general, if you don't want the deep dive on matching indexing:
- 5/6 speed derailers won't really index well
- 7-8-9-speed derailers can index their own speed and all lower ones, it's the shifter and cogs that need to match
- 10-speed and up, match all the parts together, or write to us for advice before trying to get weird with it, we have experience with mixing this stuff
Otherwise the main difference in derailers will be cage length, which affects how much chain it can take up. A larger difference between cog or chainring sizes means you want a longer cage.