Development of the Zone System is widely regarded as one of the major achievements in the history of photography. The Zone System is a method of understanding and controlling the exposure and development of the negative, and how to vary that exposure to get the results you want. The Zone System was developed by Ansel Adams and Fred Archer as a simple and straightforward method by which they could control exposure. Before Adams and Archer, the photographic industry had already standardized f-stops and shutter speeds as controls of light. But photographers still didn't know exactly how much light was needed for each individual print. Adams and Archer took the spectrum of print values, from black to white, and- using f-stops as the standard of measurement- simply assigned a Zone to each value that each f-stop of exposure produced. This results in a Zone Scale, which is a visual representation of print values from black to white.
Zones are always represented in Roman Numerals. Obviously, print values (from black to white) can be divided into as many, or as few Zones as one wishes. However, eleven is the standard by which most photographers work. This has the advantage of placing the mid-gray Zone V, the meter reading, in the middle of the scale. The Zones are numbered 0 through X. Zone 0 represents the maximum black that the print can produce. Zone X represents pure paper-base white - no image. The nine zones between are each equivalent to one increasing f-stop of exposure. Therefore, Zone III is lighter in value than Zone II, and darker in value than Zone IV, etc.