The Quaternary is a geologic period. Quaternary (an adjective meaning "fourth in order" or "composed of four items") may also refer to: Quaternary (chemistry) (see also Quaternary compound and Quaternary phase) Quaternary structure of proteins Quaternary sector of the economy, which encompasses knowledge-based services Quaternary care, health care that includes highly specialized or experimental treatments Quaternary numeral system (base-4) in mathematics Quaternary counting system, as used in some human languages Quaternary (EP), an album by Mötley Crüe See also Quinary, positional number system with base 5 Ternary (disambiguation) Tertiary (disambiguation)