What is a "Submissive Grin" in Dogs?

A submissive grin is a fairly uncommon gesture made by dogs.  It can be a bit tricky to tell the difference so people often misinterpret this signal of submission (i.e. a friendly smile)  as an aggressive snarl (exactly the opposite of the dog's intent).

A submissive grin can look like an aggressive snarl, but the two gestures are completely different in both action and intent. If you're not sure that you can tell the difference, here are a few things to look for: 

• Look at the overall body language - is it loose and happy or stiff and threatening?
• Look at the eyes - are they squinty (soft and averted) or threatening (staring and dilated)?
• What's going on in the environment? Has the dog just spotted the owner bringing the leash and is happy to be going for a walk? Or is he standing over a favorite bone stiffly (guarding it)?

Here are pics of 2 dogs. The dog on the left is doing a submissive grin and the dog on the right is doing a threat display:

Dog on left is showing full teeth with his mouth pulled back into a  "smile" - ears are gently back ("bunny ears") and eyes are squinty. This is a submissive grin.

Dog on right has her whiskers forward, hard eyes with pupils dilated, furrowed brow and a strong stare. This is a threat display.

Here is a video of a dog doing a submissive grin: http://youtu.be/Aw_1V1yguvE