The surface of the moon is heterogenous. Similar surface materials are grouped into map units by means of telescope observations, study of lunar photographs, and photometric measurements. Each map unit has lateral continuity and limited range of physiographic characteristics and optical properties (mainly albedo, the reflectivity under full moon illumination). Such units are equivalent to the rock stratigraphic formations of terrestrial geology. By application of the principles of superposition and intersection, these formations have been arranged in order of relative age and grouped into time stratigraphic units. Following terrestrial convention, the major time-stratigraphic units are designated systems; corresponding to these are periods of time. The type localities of most of the systems are in the v1cinity of Mare Imbrium. Assignment of some of the rock stratigraphic units in the Pitatus region to the systems established in the imbrium region is still uncertain.