Your dog can't talk, but his body posture tells you a lot about his state of mind. Learning to read your pet's body language is an important part of communicating with him. Canines are pack animals, so their postures are often indicative of their place within the pack hierarchy -- even if he's your only dog.
Normal posture for a contented dog is a relaxed posture. He's not trying to make himself appear large or small -- he's just right. He has a neutral look on his face, or just looks happy. He might gently wag his tail.
Meeting Other Dogs
When your dog is initially introduced to another canine, or if the two dogs haven't seen each other in a while, you see them warily circle each other, each trying to sniff the other dog's rear end. At some point, one might put his head on the strange dog's neck or give him a quick nip on the nose. One dog lies down and the other remains standing -- unless they actually start fighting. The standing dog has established that he's dominant.
If a dog wants to play, he'll initiate a play bow -- with his back end up, tail wagging and his front legs touching the ground -- inviting another dog to join in the fun. Barking usually accompanies a play bow. Your dog might also perform a play bow with you when he wants to start a game.
The top dog in a pack isn't necessarily the largest, but alpha is probably the one who can make himself appear the largest, relative to his size. When a dog assumes an assertive posture, he makes himself as big as possible. While standing tall, he flexes his musculature and holds his head high. An aggressive dog growls, raises the hackles on his neck, wags his tail slowly and deliberately, and leans forward.
A dog displaying submissive posture wants to appear as small and nonthreatening as possible. A truly submissive dog doesn't just crouch but rolls over on his back, exposing his vulnerable belly. He might urinate a little. Submissive isn't synonymous with truly scared: A frightened dog hunches his body and places his tail between his legs. He's quite tense, possibly looking to escape the source of his fear.