Dogs wag their tails to convey strong emotions, such as happiness or agitation. Other clues can help people understand whether a dog's tail wagging is a gesture of welcoming or warning.
Dogs use their tails to communicate, and people should not assume that tail wagging means the animal desires affection. The direction and speed of the wagging tail are clues to let others know whether the dog is content or upset.
Dogs usually wag their tails to the right when they are happy and to the left when they are apprehensive. A tail held high and wagging back and forth is a sign of excitement and joy. A dog that keeps his wagging tail horizontal exhibits curiosity about a person, animal or object. If the tail is wagging low to the ground, the dog is worried, stressed out or feeling ill. Dogs who wag slowly while intently gazing or growling signal aggressive feelings. Frightened or submissive dogs keep their tails, wagging or not, tucked under their hind legs.
Dogs with bobbed tails or no tails generally approach other animals with extreme caution, since they can't convey emotions by wagging. These dogs may also act aggressively toward others for protective reasons since they are unable to communicate with their tails.