Probably the most obvious, and most misunderstood, form of tail language is the wag. Even people who have spent their entire lives around dogs assume that full-throttle wags mean a dog is saying “hi” with all her might. But tail wagging can’t be interpreted this easily because dogs that are aggressive and are preparing to attack will also wag their tails.
Posted by pet editor
A wagging tail means nothing more than that a dog is feeling excited about something. Sometimes the excitement is positive, and sometimes it is negative. You have to look at the dog more closely to figure it all out.
High, stiff, and wagging = “I’m in charge”
A tail that’s upright like a mast doesn’t necessarily mean that a dog is feeling aggressive, but she’s certainly being assertive. Many people get bitten because they misinterpret stiffly wagging tails. A tail held high and wagging stiffly is the sign of a dominant dog, and it could mean that the dog is ready to attack.
Low, fast wags and short sweeps = “I’m no threat”
When a meek dog is approached by a more assertive dog—one with her tail up and stiffly wagging—she has only one reasonable response: to show the bossy dog that she isn’t a threat. Dogs who are fearful or submissive often will respond to another dog’s approach by holding their tails low and wagging them just slightly. This means, “Hey, I’m friendly and no threat at all.”