Melissa Holbrook Pierson, "What Is Your Dog Telling You? They may not use words, but dogs say a lot more than we realize with their body language", WSJ 5/11/2015:
For the same reason that Eskimos purportedly have 50 different words for snow, dogs have a vast repertoire of gestures for appeasement and propitiation. The Norwegian dog trainer Turid Rugaas has identified some 30 “calming signals”—movements offered to deflect trouble (which may also relieve stress in both giver and receiver). Supremely subtle, sometimes so quick we don’t notice them, these appeasing signals include a flick of the tongue; turning the head or gaze away; suddenly sniffing the ground or sitting; yawning; shaking off; or approaching on a curve.
[h/t Amanda Seidl, who is planning on getting a dog]