The chainline is the line along which the chain runs between the front chainrings and the sprockets mounted on the rear axle. The chainline runs parallel to the frame, across the middle chainring and the middle sprocket. If there is only an even number of chainrings and sprockets, the chainline runs between the chainrings and sprockets.
For maximum power transmission, the chainring and sprocket must lie on the chain line. Therefore, the chain line should be such that the chain runs as straight as possible in the most frequently ridden gears.