Dogs reveal so much about their mood and intentions by how they hold their tails and how they move them. We can get much more out of our daily interactions with dogs if we take the time to understand the vast amount they're telling us through body language. And that is especially true of tails. Many of us think that a wagging tail means a happy dog, but it's much more complicated than that. In fact, a wagging tail doesn't necessarily mean a dog is being friendly at all. Social context and other factors about the tail come into play to tell the entire story.

It takes a whole body to tell you what’s really going on — from ears to eyes, to lips and teeth, to stance and balance, to how the hair is raised — but the tail itself is a significant part of decoding a dog’s thoughts.

Of course, not all dogs have tails and some breeds have tails that are not as expressive, like pugs or basenjis. But all dogs do express themselves with tail movement. Here are some guidelines to read a tail for signs of what a dog is thinking.

Tail height

How a dog holds its tail speaks volumes. There is a tail held high, in a neutral position level with the spine, held low or tucked under.

A tail held high indicates a very high level of excitement. It could be joy, playfulness, alarm or any number of reasons. Whatever the reason, when your dog's tail is straight up, or even arched over the back, you can be sure your dog is highly stimulated. This is a sign that you'll want to watch your dog carefully because depending on the social situation, the dog can become overaroused and an interaction may move rapidly to a fight.

A tail held in a neutral position indicates a neutral mood. Everything is fine and dandy. Remember that different breeds have different neutral positions. For some dogs, like huskies, a neutral position is still fairly high, above the line of the spine. Yet for other dogs, like greyhounds or Rhodesian ridgebacks, a neutral position looks somewhat tucked under. It's important to know your dog's neutral position in order to get an accurate reading about whether the dog is holding it higher or lower than usual and what that means.

A tail held in a low position indicates nervousness or fear. A dog with a low or tucked tail is showing that he is unsure or afraid of what's going on — and it also indicates that the dog could become defensive and lash out in an effort to protect itself.

Speed of the tail wag

How quickly a dog wags its tail also speaks volumes, especially in combination with the tail height.

The faster a tail wags, the more excitement a dog is feeling. When the tail is held high, and is stiffly wagging back and forth at high speed, the dog is showing unfriendly intentions. This is sometimes called "flag tail" and it should be a red flag to an owner. Take this sign seriously as it could mean a fight is about to break out any second if the situation isn't diffused.

When the tail is wagging speedily at a neutral height, a dog is usually showing friendly intentions and is just really excited and happy about what's going on. But this isn't always the case. Some dogs may wag their tails quickly at a neutral height but if they are standing stiffly with a rigid body, the tail wag might not indicate friendliness at all. Take into account how relaxed the rest of the body is to gauge if a dog is definitely being friendly.

When a dog is wagging its tail at high speed while it is held low or tucked under, it is a clear indication that a dog is nervous and probably trying to be submissive, showing that the dog doesn't mean harm and would like to be on the good side of whatever it is the dog is feeling intimidated by.

A slow, steady wag at any height, especially neutral, usually indicates happiness or confidence.

A tail held stiffly, without movement, at any height is usually a sign that a dog is alert and figuring out what's going on. When a tail is held high and still, a dog should be approached carefully since he is very stimulated and is figuring out his next move. Similarly when a dog's tail is tucked and stiff, he is feeling quite fearful and should also be approached with caution since he may feel defensive. When a tail is held straight out and stiff, a dog is usually on alert and paying attention to what's going on, such as if he hears a noise or another dog barking. The dog is on alert but not necessarily feeling threatened.

Direction of the tail wag

Studies have shown that dogs wag more on the left or right side of their bodies depending on how they're feeling. A dog that wags his tail with a bias to the right side is showing a positive, happy response to whatever is happening around him. But a dog wagging his tail with a bias to the left side is showing a negative, anxious response to whatever is happening.

Though it might be a bit hard for us humans to spot if there's a bias to the left or right, other dogs see it easily and pick up on the cue. In the study, dogs shown videos of dogs wagging their tails toward the left (or negative emotion) side had increased heart rates and became more anxious. But when watching videos of dogs wagging with a bias to the right, the dogs remained neutral.

So remember, right side means positive emotions, and left side means negative emotions.

All of these pieces of information can be put together in different combinations and reveal different emotional states of a dog. By getting to know the meaning behind the various things a dog says via his tail, you can improve your communication with your dog. Even better, you can use your fluency in dog-speak to help your dog navigate social situations, understanding how he's feeling at each moment and helping him through.

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Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.