YAKKAY's Smart Two helmet is an in-mold bike helmet, which means that a PC shell is integrated with an EPS inner shell. This gives extra strength and durability, a perfect, clean surface and a very light weight. Size Medium: 51-56 cm (260g) Size Large:     55-59 cm (280g) Size X-Large: 58-61 cm (310g) Our Smart Two helmet has 6 vent holes for generous ventilation. For comfort, the helmet is equipped with self-adhesive, soft foam pads that can be placed inside the helmet as you may find it comfortable. With the helmet's adjusting system, including a discreet spin-dial neck adjuster, you can obtain a secure and comfortable fit very easily and quickly. The cheek buckles are in stainless steel and specially designed by YAKKAY. The Smart Two helmet is tested and certified in accordance with both European and American safety standards CE/EN1078 and CPSC1203. Find instructions for YAKKAY's Smart Two helmets here.