The geologic time scale (GTS) divides the history of the earth into eons, eras, periods, epochs, and ages. It is convenient for designating the relative size of a given window of history under discussion (i.e. eons are known in much more general terms than ages), or, alternatively, express the level of certainty to which an object or phenomenon can be dated.
This widely recognized dating system relies on an accepted way of interpreting geologic strata (i.e., the lower stratigraphic layers must be older). Layers of rocks were first given names, and those layers were only compared across geographic locations in 1819 by English geologist William Smith. By identifying, comparing, and sequencing fossil types, Smith established a relative sequence of strata that could be compared across geographic locations. This gave rise to the useful and universal GTS acknowledged and used today.