0 Pure black
I Near black, with slight tonality but no texture
II Textured black; the darkest part of the image in which slight detail is recorded
III Average dark materials and low values showing adequate texture
IV Average dark foliage, dark stone, or landscape shadows
V Middle gray: clear north sky; dark skin, average weathered wood
VI Average Caucasian skin; light stone; shadows on snow in sunlit landscapes
VII Very light skin; shadows in snow with acute side lighting
VIII Lightest tone with texture: textured snow
IX Slight tone without texture; glaring snow
X Pure white: light sources and specular reflections
Somewhere between Zone VI and VII is the 17% average reading that centre-weighted metering tries to achieve... It's why you occasionally saw photographers take a TTL 'incident' reading from the palm of their hand in the same light as the subject they were shooting.
This is the key bit (lifted from another source)
Exposure
A dark surface under a bright light can reflect the same amount of light as a light surface under dim light. The human eye would perceive the two as being very different but a light meter would measure only the amount of light reflected, and its recommended exposure would render either as Zone V. The Zone System provides a straightforward method for rendering these objects as the photographer desires. The key element in the scene is identified, and that element is
placed on the desired zone; the other elements in the scene then
fall where they may. With negative film, exposure often favors shadow detail; the procedure then is to:
- Visualize the darkest area of the subject in which detail is required, and place it on Zone III. The exposure for Zone III is important, because if the exposure is insufficient, the image may not have satisfactory shadow detail. If the shadow detail is not recorded at the time of exposure, nothing can be done to add it later.
- Carefully meter the area visualized as Zone III and note the meter’s recommended exposure.
- Adjust the recommended exposure so that the area is placed on Zone III rather than Zone V. To do this, use an exposure two stops less than the meter’s recommendation