The whole Earth is stratified, in a broad sense, so that all rocks and all classes of rocks—sedimentary, igneous, and metamorphic—fall within the scope of stratigraphy and of stratigraphic classification.

Rocks have many different properties, and it is possible to classify them according to any of these properties: lithology, fossil content, magnetic polarity, electrical properties, seismic response, chemical or mineralogical composition, and many others. Rocks can also be classified according to such attributes as their time of origin or their environment of genesis.

The stratigraphic position of change for any one property or attribute does not necessarily coincide with that for any other. Consequently, units based on one property do not generally coincide with units based on another, and their boundaries commonly cut across each other. It is not possible, therefore, to express all of the different properties with a single set of stratigraphic units; different sets of units are needed (see Figure 1).

At the same time, the general unity of stratigraphy should be emphasized. While many different kinds of units are needed to express the variations in all of the many different properties and attributes of the rocks, still, these units are closely related. They concern only different aspects of the same rocks, and they are intimately involved with one another in achieving the same major goals of stratigraphy—to improve our knowledge and understanding of the Earth's rock bodies and their history.