Lets us re-visit this image from the page on Lightness Organisation:
In the first image, the edges on the head at A and B are both darker than the background. In my rework these same points have undergone a a swop at edge B, with the head being lighter than the background. This technique 'wraps' the form of the head into the background. To a more dramatic degree this same trick is being used by Rembrandt in his portrait. This is a very commonly used device that has a VAST application in many different areas (landscapes, still life interiors etc). In the image below (a painting by Thomas Cole) the buildings on the left are lighter than the sky, whereas on the right the landscape is darker than the sky. This is a tone wrap.