" " It's usually a good sign when you see a cat's tail straight up. But why do cats have tails in the first place? Most cats use their tails constantly—even when they seem to be doing absolutely nothing. AdvertisementThe cat's tail is an extension of the spine, made up of 18 to 23 small bones called vertebrae. These bones, surrounded by muscle, nerves and sensitive nerve endings, give cats full control over every twist and twitch.