Ball Cap Liner Advanced Baseball and Softball Pitcher and Fielder Impact Protection Headgear Sizing: To ensure accurate fit, those with a head circumference under 21 inches should select the Small BCL (with or without temple guard); Those with a head circumference over 21 inches should select a M/L BCL (with or without temple guard) Lightweight at under 4.3 ounces and only 1/4 inch thick; ideal for baseball, softball, curling, sailing and many other sports Ground breaking impact protection material and a metal replacement outer shell; Breathable and washable cloth backing Passes NOCSAE 029 impact for front area as per ICS ballistic testing for both baseball and softball Minimalist design fits under any standard baseball cap; Adjustable strap to ensure accurate fit