The use of various methods, often multiple methods, to demonstrate the equivalency of stratigraphic units. This term refers to the relation of one stratigraphical unit to another, by petrological, osteological, lithographic, cultural, chronological, or palaeontological means. For example, stratigraphic units may be correlated using palaeontological criteria, absolute dating methods, relative dating methods, cross-dating methods, and position relative to the glacial-interglacial cycle by examining physical and biological attributes. Correlation of fossil inclusions is a principle of stratigraphy: that strata may be correlated based on the sequence and uniqueness of their floral and faunal content.