When your dog wags his tail, you probably think he’s happy. And he is, if he’s swinging it to the right; but if he’s wagging to the left, he’s likely anxious—a difference that scientists detected 6 years ago. But what about other dogs: Can they also read the message in a tail? To find out, researchers tested 43 dogs of various breeds for their ability to distinguish between wags. They monitored the dogs’ heart rates and reactions while showing them videos of other Fidos doing either left- or right-asymmetric tail wagging, as in the video above. To make sure that the animals weren’t watching other facial or body cues, the researchers also showed the dogs only a silhouetted version of the wagging pooch. In the end, it didn’t matter whether they watched the natural dog or the silhouette; every canine understood the meaning in a left or right tail wag, the team reports today in Current Biology. Those that saw the dog wag to the left looked anxious, and their heart rates increased—a sign of stress. But dogs that watched the pooch swing its tail to the right stayed calm and relaxed—an indication that right wags are an expression of companionship and confidence, the scientists say. Thus, tail wagging does matter to other dogs and is a good reflection of what is happening in a dog’s brain, the team concludes.