Based on the book by Ansel Adams, The Negative, Little, Brown and Company, 1981, Ch 4.
See Table 2, page 60 for a full description of each zone and Figure 4-4, page 54 for a B&W visual.
Spot reading taken in the Zone V image below as per page 51, NEF 12 bit RAW developed in Lightroom 1.3.
Digital Zone System? Read this http://super.nova.org/DPR/ZoneSystem/DigitalZoneSystem.pdf by Chuck Gardner.
See also 'Spot-Metering: Reading Adams in Reverse Direction' by Alex Tutubalin and Illiah Borg
here http://www.libraw.org/node/43. Note that depending on the camera's meter, the spot reading
may be 1/2 stop below 18% gray, or the midpoint of Zone V.