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 when it's damp.
One-size-fits-all the Brooks saddles we sell: B17, Flyer, B68, and Cambium's, too.
