Just like humans, dogs too convey their feelings, thoughts through the gestures and movement of their bodies besides the barking. The best way to judge a dog' social standing and mental health is through its tail. Just as humans way of expressing an emotion can vary, so can the dogs' tail position; for example, a relaxed tail position of various breeds differs; the greyhound positions its tail lower than the feathery tail of golden retriever which is still lower than the white West Highland terrier.
If the dog tail is parallel, yet not rigid and points away from it, it shows that the dog needs attention and something interesting is going on. If the dog meets someone new, stranger or intruder, its tail points away horizontally; which means that it is ready to take up the challenge of judging "who is the boss around?"
A dog which is dominant or is "the boss" dog is easily noticeable, through its upward (between vertical and horizontal positions) tail. Moreover, if the dominant dog feels confident and in control, its upright tail shall is a little curved above the back.
Generally, the pose of the dog is with have its tail down close to its rear legs. If the dog is at rest and calm, then its tail shall be down, below the horizontal position and away from its legs. If the dog is sad or upset, it keeps its tail down and brushes it slowly backward and forward, while its legs are straight. However, if the dogs' legs are slightly bent towards it and its' back bend sloping downward; the dog is insecure and anxious. This happens when the dog finds itself in an unfamiliar situation.
Whenever you see the dog having its tail between its legs, it means it is scared, frightened and is pleading for mercy. This occurs whenever a dominant human or dog is in its vicinity. Hence, it is being submissive and accepts the other dog or human superiority.
If the hair of the dog stiffens or bristles, it is ready to fight the moment the other (generally dog) is. The tail may be in any position. However, if the bristling tail is a little up and above the back, the dog is ready to fight to show its leadership and is not scared.
Dog breeds like German shepherds look like wolves and their tails resemble the wolves which are held high and has a sharp bend. This is similar to a bristling tail and could mean possible hostile behavior.
If a dog likes anyone or anything, it is happy and wags its tail madly and broadly. If they are playing with another dog and looking aggressive with its growls, barks and activities, they wag their tails to reassure that all is okay and is simply out of fun. Besides, during training, when the dog is trying to understand what the trainer is trying to teach or do, its tail positions itself half-mast and wags slowly. The moment it finds the solution, it wags faster and furiously.