Anyone who spends a lot of time with dogs should understand dog language. Dogs don't communicate with words, just body language. Understanding the way they communicate can help you avoid conflicts between dogs in your home or with a strange dog.
Friendly Greetings
A friendly dog greeting does not necessarily begin face-to-face. Most dogs will actually approach each other along the side first, sniffing the sides and rear. Many conflicts begin because owners introduce their dogs face-to-face. Instead, make an arch away from the other dog and let your dogs approach each other in a more friendly way.
When a dog is excited to greet, he will have relaxed body posture. His mouth will be open loosely and his ears pointed forward. His body should be slightly low and wiggly. His tail will be wagging loosely in a relaxed circle, not high and straight. A dog will indicate that he wants to play by lowering his front half to the ground while leaving his butt in the air.
Warning Signals
Dogs also have several signals that indicate they are not happy to greet. Being aware of these signals will help prevent problems.
A fearful dog will have his tail tucked between his legs and ears back. His pupils will be dilated, often so much they appear completely black, and you will be able to see the whites of his eyes. His body posture will be leaning away from you and low to the ground. He may try to hide.
A dog who may become aggressive will usually have an offensive body posture, leaning forward stiffly. His ears may be up, and his eyes might appear normal or dilated. His tail may be wagging, but it will be stiff and slow, often held high.
The first aggressive warning will be a momentary freeze in a stiff position. At this point, the eyes will dilate, and the tail will be stiff or tucked. This is usually followed by baring teeth, growling or snapping. It could escalate to a bite.
Barking
Often, barking (at humans) is used for attention or play, which your dog's will demonstrate with a relaxed body posture. Dogs playing may bark or growl frequently, but they will use play signals. If you are worried, pull away the dog that appears to be "winning." If the other dog runs back to play, let them continue. If he runs away, separate them for awhile.
Barking can also be fearful. If your dog is charging and barking while low to the ground and leaning back, alternating between charging and running, he is exhibiting fear. Remove him from the situation. High-pitched barking is also an indication of fear. If your dog's body posture is forward and stiff with fierce barking, this can be offensive, and the other dog could be in danger.
Body language will vary a little from dog to dog. It's important to be familiar with your individual dog's signals and be able to identify troublesome signals from unfamiliar dogs.