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.”
Low, slow wags and large sweeps = “It’s good to see you”
Sometimes a wag is really just a wag. You’ll know your dog is wagging because she’s happy to see you and wants to play when her tail is low or even with her body and is wagging a little slowly, but in wide sweeps back and forth. Usually, her whole butt will be wiggling as well.
Keep in mind: Some hunting dogs, like spaniels, have been bred to wag their tails constantly while pursuing quarry. To signal that they’ve found their game, they stop wagging and hold their tails stiffly up in the air. On the other hand, herding dogs are bred to have less tail movement so that their wagging tails don’t distract or excite the animals that they’re trying to herd. If you didn’t know that about these animals, you might think one was overly excited while the other was overly staid, when it’s really just the way they were bred.