Researchers in Italy conducted a study on the directional motion of dogs' tails and found that it means more than the desire for a treat.
According to the study findings, a tail that moves in the rightward direction indicates positive emotions. However, a tail that moves leftward indicates feelings of anxiety, fright or anger.
In addition, researchers also found that dogs play off of tail wagging. Therefore, if one dog sees another's tail move to the right, he becomes more relaxed.