The lowest-ranking chronostratigraphic unit. The duration of a chronozone is defined by a type section which may be based on a biostratigraphic zone (in which case it would include all rocks laid down during that time interval regardless of fossil content or rock type), or it may be defined on the basis of the time span of an existing lithostratigraphic unit. With increasing knowledge, chronozone boundaries may be found not to coincide with the boundaries of the stages to which they belong, and stages may more precisely be divided into substages. The geologic-time unit equivalent to a chronozone is a chron.