Brooks calls this a rain cover and while it's perfectly good for that, it's also a good idea to use when it's not raining. Leather doesn't know the difference between rain and your moisture, and your saddle will get hammock-y and won't last as long if you use it a lot when it gets wet.
One-size-fits-all the Brooks saddles we sell: B17, Flyer, B68, and Cambium's, too. The Randi-Jo saddle covers fit, and look better because she makes them in a couple sizes, and adds elastic butt these Brooks covers do the job for half the price. If you're really on a budget just use a plastic bag from the supermarket.
