In my peripheral vision I see blanket movement, I glance over to witness more motion, it is Luke's tail, he is dreaming. Whatever he is dreaming about he is wagging, but a wagging tail does not always mean happy or a good thing. I remember several years ago when I was out walking on a beach alone when a stray dog came out charging. It stopped about 20 feet away barking menacingly, growling and wagging just the very tip of his tail which was curled well over his back. Hmmm, charming.
I have to admit I love a dog that is a real wagger. That said, there is a reason why I like a wagging tail, and that is because it makes reading the dog much easier. Reading a dog is a whole package process but the tail is a huge part, if they use their tail a lot. Some dogs like my Tilley girl are not big waggers and if she is not super happy then it just sort of hangs there and doesn't say much. And tails can have a lot to say.
When you read a tail there are many things to watch for, position, speed, tension and the rest of the body. For some dogs like my Luke, they hold their tail naturally high. He is a fairly dominant but friendly boy, the only time it freezes in its upright position is when he is making a point. It is essential to look at the tension of a wag. The tail could be wagging loosely like a flag in the breeze or very taught and fast. Loose is good; the more relaxed a dog is the more flag like their tail will be.
This is one of the reasons that I am very against docking tails. Dogs communicate hugely with each other with their tail and to chop it off handicaps them to a degree. The following link is to an article written with regard to research done in British Columbia, Canada on length of tail and intra-species communications. It is a long read but extremely interesting.
Behavior response to different tail lengths
A wagging tail can be very deceptive, first decipher what the tail is saying, then look at the rest of the communication being delivered via the entire body. Many people have been bitten to their shock by a dog wagging their tail.