The Brooks saddle cover is made from nylon and designed to protect your saddle from the elements in inclement weather conditions.
It has a draw cord for tightening adjustment.

Sizes:
Medium 340 x 180mm
Large 350 x 240mm

Medium fits most saddles, such as the B17, B72, Professional, Finesse, Swift, etc.
Large fits the wider saddles B66, B67, B33, etc.