If a One K™ Helmet does not come in your size, you can easily change the liner for a perfect fit. Our liners also provide ultimate flexibility for growing children, shared helmets, or women who wear hair differently for schooling and show! Three shells make up all sizes. The base shells are Medium, Large and X-Large. Use the Medium size helmet with a Small or X-Small liner to change the fit of the Medium helmet into a Small or X-Small. The Large shell will only accomodate a Large liner and cannot be size-adjusted but having an extra liner on hand is always nice. Use the X-Large size helmet with the XX-Large liner to change the fit to XX-Large.