Lazer O2 Helmet Patented Rollsys fit system with vertically adjustable head basket Integrated Rigidity Brace offers superior resistance to impacts Optional tail light Works well with Aeroshell, Insect Mesh, Cappuccino Lock and Lazer Eyewear Features 24 ventsSmall Fits: 52-56cmMedium Fits: 55-59cmLarge fits 58-61cm