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The distance between the bicycle centerline and the center of the crank. Can also refer to the distance to the center of the cassette. A drivetrain will be designed around a certain chainline. The advent of disc brakes has caused drop bar bike chainlines to move outward slightly.

Chainline is the amount of horizontal offset between cogs on the chainring and cogs on the cassette.

Various chains are designed to accommodate different levels of offset and more offset can cause friction in the chain.