The Charles Owen Luna Riding Hat Liners are designed to fit the Charles Owen Luna Riding Hats. They can be used to refresh your helmet and lengthen its lifespan. They are easily clipped into the hat, making it simple to remove and wash when needed. These Charles Owen Hats are bought in three shell sizes (S, M, L). You then choose the hat liner in your "cm" hat size (measured around the widest circumference of the head), this creates the size of the helmet. On each shell, you can interchange the liners to create a different size. We also recommend purchasing a spare liner. This means that you can use the spare whilst washing the other or take the two liners to a stay away shows for a fresh head each time you ride. One liner is free when purchased with a Luna riding hat.