A new study shows how a dog's mood can be partly determined by its tail wagging.
That wagging, scientists say, can be used to communicate to other dogs.
An earlier study by the same researchers at the University of Trento found that a wag to the left shows negative emotions, while a wag to the right indicates positive ones in dogs.
Now they say that not only does that still hold true, but also that other dogs notice that too.
Researchers used videos of dogs or silhouettes to analyze the tail movements.
They used 43 dogs of varying varieties, including beagles, boxers, border collies and German shepherds, among others.
More from GlobalPost: Economists are horrible, horrible people. So says science
The study had observer dogs watch the videos and silhouettes, while scientists analyzed their responses.
When the dogs wagged to the left, the observer dogs had faster heartbeats and they showed higher degrees of stress.
When the dogs wagged their tails toward the right side of their bodies or did not wag at all, observer dogs remained calm.
"The direction of tail wagging does in fact matter, and it matters in a way that matches hemispheric activation," said study author Giorgio Vallortigara.
"In other words, a dog looking to a dog wagging with a bias to the right side—and thus showing left-hemisphere activation as if it was experiencing some sort of positive/approach response—would also produce relaxed responses. That is amazing, I think."
The lesson here? If a dog is wagging its tail more to the left, it doesn't like you.
The study was published in the journal Current Biology.