Understanding Edge Flow

In the past I've submitted front-view face edge loop images, and they can be helpful if you want to model that specific face, and you can try and copy that edge flow for your own original plans as well, but it doesn't necessary help you to /understand/ what's going on, so that you can better create your own edge flow for a face.

So I threw this together. The 'edges' shown here aren't the exact edges that you'd use, but rather a guide of what general area and shape you'd try to put edges into.

I also color coded three patches that are important.

The 'mask' around the eyes.
The flow down the side of the nose, coming from the eyes.
The flow coming from the nostril down and around the mouth.

If you can get those areas edged out nicely, you'll get the best deformation when trying to animate your face for expressions and lip sync.