Chron and Chronozone
A chron is the geologic time corresponding to a chronozone. A chronozone is the rock that is formed within a time interval of an extended biological zone. A chronozone is a formal chronostratigraphic unit at the level below a stage. Another interpretation is that chrons and chronozones are informal or freely designated units. For example, a magnetostratigraphic unit or stratigraphic unit can be subdivided into corresponding chrons and chronozones.