Built with an aerospace shell design & materials, Troy Lee were able to get the D4 Composite helmet to 1050 grams without sacrificing any strength. MIPS (multi-directional impact protection system) is used to add an additional layer of safety in the event of a fall. For comfort, the D4 Comp routes cooling air through a front air intake system that flows through the polymer centre & power grid side mesh keeping your head cooler during the ride. X-static & XT2 anti-microbial/anti-odor padding keep the inside of the helmet a much nicer place to be. Size Guide: X Small = 20.8-21.2" (53cm-54cm) Small = 21.6-22" (55cm-56cm) Medium = 22.4-22.8" (57cm-58cm) Large = 22.8"-23.2" (58cm-59cm) X Large = 23.6"-24.4" (60cm-62cm) XX Large = 24.4"-24.8" (62cm-63cm)