SUMMARYFrom smallest to largest, the component layers of a sedimentary body are laminae, laminasets, beds and bedsets. Different arrangements of these layers characterize different types of sedimentary bodies and identify different depositional processes. Concepts of these layers are redescribed because previous definitions are not adequate for modern quantitative descriptions of sedimentary bodies. The four kinds of layers are genetically similar; when compared with each other, they differ principally in areal extent and interval of time for formation. Because beds are usually the most readily recognized layers, they are considered the basic “building blocks” of sedimentary bodies.Beds are bounded by depositional surfaces termed bedding surfaces; each surface is practically synchronous, and a bed can be considered an informal time stratigraphic unit of limited areal extent and of relatively short time span. This concept broadens the scope of intrabasinal correlations, permitting time correlations more refined than possible using fossils or radioactive age dating. Also, a better understanding of the mode of genesis and recognition of distinguishing characteristics of different types of sedimentary bodies follow from this conc