Comments are given on a number of papers dealing with principles and procedure in stratigraphic classification which have appeared in British journals during 1963–65. It is emphasized that a clear distinction should be made between biostratigraphic units—bodies of rock strata characterized by certain palaeontological features and bounded by the limits of known occurrence of such features—and chronostratigraphic units—bodies of rock strata characterized by representing the rocks formed during certain time-spans of earth history and bounded above and below by isochronous surfaces. Biostratigraphic zones and the chronostratigraphic terms chronozone and stage are discussed.