Durable ABS hard shell for protection and EPS foam liner for security;Retention system provides a secure and snug fit;Easy-to-use magnetic Fidlock buckle;Size small fits circumference48 cm - 54 cm and hat sizes 6 1/2 - 6 3/4;Size medium fitscircumference 55 cm - 58 cm and hat sizes 6 7/8 - 7 1/4;The size large fits circumference 59 cm 61 cm and 7 3/8 7 5/8;Small size generally fits children while medium and large sizes fit adults;