The Scale of Zones
At this point we have established the midpoint on a scale of exposures (Zone V), and negative values and print values (Value V).  We also know from experience that reducing exposure will produce a darker print value, and increasing exposure will produce a lighter print value.  To determine the remainder of the scale, then, we define a one-stop exposure change as a change of one zone on the exposure scale, and the resulting gray in the print is considered one value higher or lower on the print scale.
Thus, making a reading from a single-luminance surface and reducing the exposure by one stop gives Zone IV exposure, and yields print value IV, which is darker than middle gray.  Further reduction at one-stop intervals gives exposures of Zones III, II, I, and 0, with the corresponding darker tones in the print of Values III, II, I, and 0.  (Each of these, of course, also has its corresponding negative-desnsity value as the intermediate step.)  Similarly, if we increase the exposure one stop at a time from Zone V, we have Zones VI, VII, VIII, IX, and X.
Ansel Adams, The Negative (1981, 49-51)