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Chainline
From WikiPedalia
This refers to how straight the chain runs between the front and rear sprockets. Ideally, both sprockets should be in the same plane, so that there is no sideward motion or stress to the chain. This constitutes "perfect chainline".
In the case of derailer geared bicycles, the chainline is not perfect in most gears. The worse the chainline, the worse the mechanical efficiency of the drive train.
"Correct" chainline for a derailer system is a matter of opinion, and depends on the intended use of the bicycle. There are two "simple" answers to the question of what constitutes proper chainline:
Also see my article on chainline
