This lycra stretch hat silk from Charles Owen is designed to be worn with the Charles Owen Pro II Plus Jockey Helmet. 

This hat silk stretches to offer a smooth, tidy fit and is designed with a mesh ventilation panel.

Sizing:

The size of hat silk corresponds to the size of your helmet:

Small: 52-53cm
Medium: 54-56cm
Medium 1: 57-59cm and all Round Fit sizes
Large: 60-61cm
X Large: 62-65cm