Lazer O2 Celeste Unisize Rollsys retention system Rigidity Brace System (RBS) adds reinforcement in the event of multiple impacts Internal ventilation channels for optimum airflow and Coolmax padding Available in two size options: Unisize for 53-61cm heads and and oversized (XL/XXL) for 61-64cm heads; check list of attributes below for size range in centimeters In-Mold (2 piece) construction and 24 vents