A "dewclaw" is a vestigial digit on some mammal's legs. It grows higher on the leg so that, when the animal is standing, it does not make contact with the ground.

Dogs

Dogs almost always have dewclaws on the front legs and occasionally on the hind legs. There is some debate about whether the dewclaw helps dogs to gain traction when they run because, in some dogs, the dewclaw makes contact when they are running and the nail on the dewclaw often wears down in the same way that the toenails on their other toes do, from friction with running surfaces. However, in many dogs, the dewclaws never make contact with the ground; in this case, the dewclaw's nail never wears away, and the nail needs frequent trimming to prevent it from becoming so long that it easily catches on things.

There is also some debate as to whether the dewclaws should be surgically removed. Because they are a weak digit, barely attached to the leg, dewclaws can rip partway off, which can be painful and prone to infection. This is somewhat more common if the nail is allowed to grow excessively. However, for those dogs whose dewclaws make contact with the ground, it is possible that removing them could be a disadvantge for a dog's speed in running and changing direction, particularly in performance dog sports such as dog agility.