Giro Merino Wool Ear Flaps Under Helmet Cycling Cap Our Merino Wool Cap is thin enough to fit under helmets, with an updated panel construction that reduces bunching, too. We've also shortened the brim compared to some caps, for better upper peripheral vision without reducing the ability to block sun and rain. CONSTRUCTION: Winter weight Australian merino wool blend X-Static liner Reinforced brim Ear flaps fold up or down Small/Medium, 51-57cm Large/X Large, 57-63cm