Just like the Record equivalent, it's 5.9mm wide to enhance the quietness of your drivetrain and to make your rear derailleur easier to adjust. In fact, the lightweight links between the Record Ultra Narrow and Chorus Ultra Narrow are identical. They even have the same Ni-PTFE finish. All that separates them is the links themselves. The Record chain has hollow links, while the Chorus links are solid. The net difference in weight is 14g. The net difference in shifting quality is nil. Like the Record Ultra Narrow, the Chorus version uses an HD-link closure system.