I was nervous about buying the small size muzzle for my chug, because so many people seemed to have trouble with the small size being too small for their dogs. Her head, measured from the base of her jaw and around behind her ears was 12" right on the high end allowed by the small size. But I've had problems with sizes being too big on her before, since she's rather more petite and narrow than a 100% pug, so I went for it.This muzzle fits her perfectly, in fact, I had to tighten it up a bit. If your dog measures on the high end of the size allowance (12") and has a much wider face than my dog, though, I would get the medium, to give your dog's face a little more room. As I said, her face is very flat, but more narrow and delicate than that of a 100% pug.She can see, drink, lick, and pant with the muzzle on, and it allows me to treat her through the hole in the front. She can't eat or bite with the muzzle on.She doesn't seem to mind it very much, but she can get the muzzle off if she is left unattended for more than a few minutes, but muzzles are not meant to be worn without supervision, so that doesn't bother me.I have included some pictures of the fit, the shape of my dog's head, and the handy diagram on the back of the package that shows how to measure for the fit. Sorry for the bad image quality, the camera on my phone is not good.