On tail wags

The height, speed and nature of the wag all say something...

A low, loose, sweeping wag is often a relaxed, social dog

A low, rapid wag often signals submissive or social appeasement, though it can also reflect anxiety or conflict

A tail carried above the line of the spine indicates arousal, the dog is alert, engaged or excited

A high, stiff, slowly flagging tail is a warning signal, indicating confidence, a challenge or threat behaviour

And studies have shown a tail wagging to the right reflects a more positive emotional state, while a wag to the left suggests the dog is facing something negative.

Reading the tail by itself is never enough, it's one part of the conversation a dog is having.