Sizing Notes: Please be aware that this style runs a half size smaller than the standard Slip On. The Classic Slip On Mule, a retro take on the iconic slip on, combines the iconic look of the Classic Slip On Skool with the casual, easy on-and-off styling of a mule. Featuring canvas uppers, the Slip On Mule also includes re-enforced toecaps and signature rubber waffle outsoles.