There's a maximum teeth differential. I think it's a combination of front/rear.
The longer the rear mech cage, the greater the ability to take up chain slack in looooow ratios (relative to your highest ratio).
Usually: long for triple, middle for compact (I think), and short for standard double, unless you're running a 27/28t (maybe?).
@BringMeMyFix started
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There's a maximum teeth differential. I think it's a combination of front/rear.
The longer the rear mech cage, the greater the ability to take up chain slack in looooow ratios (relative to your highest ratio).
Usually: long for triple, middle for compact (I think), and short for standard double, unless you're running a 27/28t (maybe?).