Chain line is the distance (in mm) between a virtual line from the frame center and the point right between the two chain rings.
Or the one chainring if is a 1x11s, 1x12s or 1x12s groupset.
Shimano 10-speed and 11-speed road front derailleurs are designed with a 43.5mm chain line.
Newer 12-speed road front derailleurs have 44.5mm chain line.
Shimano gravel Di2 groupset (GRX) is 47mm.
For larger tires on gravel bicycles you may need larger chain line.
