Dog yawning is probably the most amusing signal. It is successfully and frequently used not only by dogs, but also by people. A dog yawns in many various situations. For example, when it is taken to a vet clinic, when there is a quarrel at home, when a child is embracing a dog, when it is made to be placed too close to some object or a person.
If a dog feels anxiety and fear, unsure in itself, feels stress, and you want to calm it down, you may start yawning too.
If a dog yawns, another dog will start yawning as well.
Example. A dog starts worrying when somebody is running or playing around. When I feel that a dog is anxious, I am fading and start yawning. It makes a dog calm down.