basically, chain line is a parallel line to the centerline of the bicycle ..
Back is the center of the cassette/freewhee; 3 of 5 4 of 7 etc.
or the gap between the 2 in the center for even numbers..

the front, the middle chainring on the triple should line up with the back..

A double is set up the gap between the 2, (an even number) is on the chain line..
so the BB spindle has to be longer (on the right end)






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