Perfect play tails.
One of the biggest misconceptions in dogs, is wagging tails. "Oh look, he's wagging," is taken as a sure fire sign that he's friendly. It could mean that he is friendly but it could also mean just the opposite. Dogs wag for all different reasons; if you watch a tail closely you will see that it has many different wags to it. The way a tail is held, the speed in which it is being wagged, the part of the tail being wagged and of course the rest of body all play roles in understanding the what a wag means.
Of course a wagging tail is only as good as the dog or person reading it. As I said many people misunderstand a wagging tail as friendly and approach when they see it. Sadly many bites occur because people don't read dog tails correctly. You need to really watch tails for a while to learn their different communications; that and see what the rest of the dog's body is doing as well.
Tails signal change insanely quickly; and if you are not watching you might miss what the dog is saying. Watching a pack of dogs interact is so interesting; their signaling to each other is done with lightening speed and precision. Watching Elsa, Luke and Penny interact is a great display of tail signaling. One second Penny has her tail held high and vibrating as she prepares to launch herself at Elsa; the next it is slung very low and speed wagging as she slowly approaches the man of the pack, Luke.
I really hate when I cannot see tails; they give us so much information. Some breeds have docked tails which put both the dog and us at a disadvantage as far as reading. Others have tails that are curled up over their back which are very hard reads. I find that many of these curled tail dogs tend to be dogs that other non curled tail dogs are wary of. I know that both Luke and Elsa are very cautious around them and tend to just stay away. They are a hard read; that tail stays up there on their back and barely moves.
Elsa has a very fluid tail; obviously I love it. I get to see things that I have never seen in any of my own dogs before. As you approach her relaxing on the couch; she squints and slowly wags the 1/2 end of her tail. Sometimes it's just the last two inches that barely twitch; but there is enough movement to visualize a reaction on approach. Her tail is almost continually in motion; it is always saying something. Often it is only her tail that gets my attention as I may be on my computer and hear the thumping of it as she gets goofy trying to draw me from my work.
You must look at the posture of a tail as well as the wag. The higher a tail is held the more confident a dog is. The wag or movement will tell the rest of the story. Luke was commonly under fire in his younger years as he was an extremely confident guy. He would enter a park with his tail held high charging around; much to the disapproval of other males or dominant females in the park. He was not dominant, just very confident. But entering the park so cocky got him into trouble
Tails have a lot to say; do not let a wagging tail fool you.