Everyone knows that a wagging tail means a happy dog, right?

Nope. What “everyone knows” is not always true. Dog say a lot with their tails. Sometimes, they are showing happiness, playfulness, and friendliness. Sometimes the tail wag means the exact opposite. How can you tell?

Koala is wagging her entire tail in the video above. She’s excited about something (Deni is probably fixing her dinner!). If she were standing, the tail would flow down from her back and the wag would extend widely to both sides of her body. This is a happy, friendly, excited tail wag.

A tail held level with the body or a little lower, wagging quickly, usually means good things. The dog is happy or excited. When a dog is really happy, she might wag her whole body. This is known as the tail wagging the dog.

When Cali is greeting someone she adores (by which I mean “someone she has met before”), her entire body wags and wriggles with joy. Not all dogs are that exuberant, but it’s usually possible to see signs of happiness other than the waggy tail: soft, open facial expression, bright eyes, a smile, that wriggly body. In Cali’s case, this can be challenging to distinguish from her whole-tail happy wag when she spies someone she hasn’t yet become best friends with. The chance to meet someone new is almost as great as seeing someone adored, so she wags high and fast then too. But maybe a little less of the body wag …

The tail gets higher as the dog becomes more aroused … or more assertive. A tail held high radiates confidence but could also indicate aggression. If you’re approached by a dog you don’t know with tail straight up, be cautious, especially if the tail is doing more of a vibration than a wag.

An exception is the “helicopter tail,” a maneuver sometimes performed by Labs and Goldens when they are playing ball. I am sure there are other times (and other dogs) where the helicopter move shows up. It’s a high tail that make complete circles over the dog’s back, like a helicopter rotor.

A slow thump often indicates a happy, relaxed dog. But a slow wag with the tail low, when the dog is standing, could mean the dog is checking out something that makes her feel anxious or unsure. The narrower the sweep of the wag, the more nervous the dog. People often mistake a slow, conservative wag for a welcome and move in to pet the dog. This can be a mistake; an anxious dog who feels crowded or threatened could respond badly. I’d let a dog showing this nervousness have her space, and I wouldn’t make direct eye contact. If, after sniffing me, she wanted to approach, great! But I wouldn’t push the issue. A tail held very low or even tucked signifies submission or nervousness; it might move a bit but, this is not really a tail wag, even if the tail is moving.

As it dawns on a dog that the anticipated good thing is not going to happen or will be delayed, the excited, happy wag could slow into the uncertain or anxious wag; this is one of many ways that dogs converse with us using their tails.

There’s more to a tail wag than tail height and width of the wag. A study done in Italy a few years ago found that dogs wag more to the right when they see someone they know and more to the left if they are nervous. The study also determined that dogs can read that nervousness in other dogs’ tail wags and are likely to respond by becoming anxious when they saw left-wagging dogs.

These whole-tail wags are easy enough to read once you practice a bit. But sometimes dogs say much more with their tails. Let’s look at Koala’s unique tail-speak:

I’ve never seen another dog do this exactly, but the partial tail wag is great communication. Koala’s thinking about something: she might be listening hard to what Deni is saying and trying to understand; she might be hoping that Deni is about to give her a wonderful treat and watching closely to see if she can tell. This tail-tip wag means that Koala is thinking about something, solving a problem. But it’s a good problem; she’s tentatively, hopefully, anticipating something good.

So, pay attention to what your dog’s tail is telling you. It’s a great way to read her mood and improve your two-way conversation.