Tail position is an important indicator of social standing and mental state of a dog. There will be some variations, of course, depending upon the natural tail position of the dog: a West Highland white terrier will carry its carrot-shaped tail higher than a golden retriever its flowing, feathery tail, and a greyhound’s relaxed tail position is lower yet.
Almost horizontal, pointing away from the dog but not stiff: This is a sign of attention. It roughly translates as “Something interesting may be happening here.”
Straight out horizontally, pointing away from the dog: This is part of an initial challenge when meeting a stranger or an intruder. It roughly translates as “Let’s establish who’s boss here.”
Tail up, between the horizontal and vertical position: This is the sign of a dominant dog, or one who is asserting dominance, and translates as “I’m boss here.”
Tail up and slightly curved over the back: “I’m top dog,” this says. It is the expression of a confident, dominant dog who feels in control.
Tail held lower than the horizontal but still some distance from the legs: “I’m relaxed.” “All is well.”
Tail down, near hind legs: This changes its meaning with the posture of the dog. If the legs are still straight and the tail slightly brushes back and forth, it means “I’m not feeling well” or “I’m a bit depressed.” If the legs are slightly bent inward, giving a slight downward slope to the back it means “I’m feeling a bit insecure,” especially in an unknown setting or situation.
Tail tucked between the legs: “I’m frightened!” or “Don’t hurt me!” This is especially common in the presence of a dominant dog or person, when it can also mean, “I accept my lowly role in the pack, and I’m not trying to challenge you.
Bristling hair down the tail: The bristle in the dog’s tail is a sign of aggression. It may modify any tail position. Thus, with the straight out it means “I’m ready to fight if you are!” and with the tail slightly up or over the back it means “I’m not afraid of you and will fight to prove
Crick or sharp bend in the tail when held high: This is more characteristic of the dogs that look like wolves, such as the German shepherds, and means much the same as the tail bristling. It is the precursor to possible aggression.
Broad tail wag: “I like you.” This is often shown during play, when one dog seems to be attacking the other, pouncing, growling, and barking. The wagging tail serves as reassurance that this is all in fun. It also means “I’m pleased” in many contexts.
Slow tail wag, with tail at half-mast: During dog training, I interpret this as “I’m trying to understand you. I want to know what you mean, but I just can’t quite figure it out.” When the dog finally solves the problem, the speed and size of the tail wags will usually markedly increase.