There are five major time units on the geologic time scale. The units are—in order of descending size—eons, eras, periods, epochs, and stages (although some list this division as ages and subages). The eon represents the longest geologic unit on the scale; an era is a division of time smaller than the eon, and is normally subdivided into two or more periods. An epoch is a subdivision of a period; a stage is a subdivision of an epoch.