Drive links
The toothed links on the underside of the chain that engage the drive sprocket and run in the bar groove. Their count is what determines the chain's overall length for a given bar.
Bar length in centimetres or inches is not enough to order a chain, because bars of the same nominal length from different makers can take different link counts. The drive link count is the exact figure, and it is normally stamped on the bar along with pitch and gauge, or can be counted by hand on the old chain. Drive links also carry oil into the bar groove and clear debris from it, which is why a chain full of packed sawdust lubricates poorly. Bumper drive links — taller links between cutters — are a low-kickback feature on many consumer chains.