One K helmet liners provide flexibility for growing children, shared helmets or riders who wear hair differently for schooling and show! The liners are a perfect solution for sweaty helmets, just swap out liner for a fresh, clean and dry one. THREE SHELLS MAKE UP ALL SIZES. XSmall, Small and Medium Helmets are built on a Medium Shell - Use XSmall or Small liner to size down. Use Medium liner for Medium size fit. Large Helmets are built on a Large Shell-Use Large liner only. XLarge helmets are built on a XLarge Shell - Use XLarge liner for XLarge fit. Use XXLarge liner to size up.