A nail file designed for dog nails will put on the finishing touch. If left to grow too long, the dog's nail will cause the toe to turn up and the dog will not walk on the foot properly. If you hear the nail clicking on the floor, it is time to clip. Nails that are too long can catch on things such as fabric and the struggling dog can hurt himself. Additionally, dogs with long nails can scratch their human companions.
Many dogs have a dew claw, that is, an extra toe and nail, higher up on the leg on the inner side.
These claws do not touch the ground and therefore do not get worn down at all. They can grow quickly and if left unclipped can catch on things causing harm. The nail can grow completely around if left unchecked. If you inadvertently cut the nail, the dog will feel pain and will bleed. If you are lucky enough to have a dog with white nails you can easily see the quick, but if your dog has dark nails you will have to gauge how far down the quick goes.