A happy dog that is busy interacting or exploring will hold his tail high and wag it horizontally. When he stops to check something out, the tail drops and stops wagging.
If two dogs meet, the dominant dog will hold her tail high and the submissive dog will hold her tail lower and may even tuck it between her legs.
If a dog is threatened, he’ll hold is hail high, either straight up or even curled over his back. The tail will be still or may vibrate, especially if two dogs are meeting and don’t yet know which one is dominant.
When your dog’s tail is tucked between her legs, she’s scared or uncertain. (My dog Teddy used to tuck it so far under that it laid against her stomach!)
A female dog in heat will hold her tail off to one side.
What have you noticed about your dog’s tail position?
Information from Dog Behavior: An Owner’s Guide to a Happy Healthy Pet.