Rear derailleurs have 3 significant specifications:
1. What's the biggest cog they will handle.
2. How much chain slack will they take up.
3. The amount of cable pull required to make an indexed shift.
Mountain rear derailleurs will usually handle a bigger rear cog and take up more chain slack. The cable pull thing is kind of a moving target.
Front derailleurs have a BUNCH of specifications and everything has to match. On the front, friction shifting is your friend.
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