Practical considerations indicate a need for the recognition of three fundamentally differing kinds of lithostratigraphic units: (1) group, formation, member, et cetera, which are traditionally designated on the basis of their position in vertical sequence; (2) lithofacies, which are lateral quantitative variants; and (3) mutually intertongued bodies or lithosomes, which are segregated on the basis of both their vertical and lateral position.

Past usage of these terms has been neither consistent nor always logical. Each is useful for specific purposes, and each illustrates relations not shown by the other two.

The three rock-stratigraphic categories are defined, and the implications of their distinction and usage are discussed. Tongue is shown to be a lithosomal appendage. Lithotope is shown to be a non-stratigraphic, sedimentological term, and lithostrome is defined as its stratigraphic counterpart.