Dogs use their tails for communication and for balance during sharp turns or fast maneuvers. Tail wagging also may help dogs spread their individual scents around as a way of marking territory.
Communication is the most common purpose of a dog's tail. The way the animal carries its tail gives insight into its emotional state. A high, rigid, quickly moving tail means the dog is dominant and aggressive, while a low tail means the dog is fearful or unsure. A level, wagging tail means the dog is happy and relaxed. Working dogs may hold their tails horizontal but rigid when they are focused and confident.
A dog's tail is an extension of its spinal column. Because of this, dogs who work fast and need to turn sharply, such as herding dogs and sighthounds, use their tails to help counterbalance the weight of their body and keep from falling over. This allows them to follow their prey without losing valuable time slowing down for turns. Dogs who work in water, such as Labrador retrievers and Newfoundland dogs, use their tails like a rudder to help control direction.
Tails also help dogs mark areas with their scent. Dogs have anal glands that secrete a unique scent, and when they wag their tails some of this scent is released and fanned out into the air.