Bern Berkeley Helmet with Black Knit 20 1/2 - 21 7/8 Meets Safety Certification ASTM F 2040, CPSC and EN 1269 Berns exclusive Zipmold foam is a liquid foam injection process that provides a better weight to strength ratio so Bern can provide customers a thinner helmet. Fit system at back of the head customizes to the shape of the head, keeping it stable and comfortable. Compatible Bike/Skate/Warm Weather liner available (sold separately)