ABSTRACT: Relative time in earth history is linked to geometrical relationships —below means older, and above means younger—and is rooted in the three fundamental principles of horizontality, lateral continuity and superposition enunciated by Steno (1669). This relationship between spatial geometry and time is the reason for dual terminology in chronostratigraphy, with rock-units (Lower/Upper) determining time-units (Early/Late). Abandoning the former would sever chronostratigraphy from its roots, and lead to semantic confusion. Lower and Upper specify position, and apply only to the observed sedimentary record. Early and Late specify interpreted time, and denote not only relative age (i.e., time of deposition) but also time of subsequent diagenesis or alteration. Although each concept closely implies the other, they are not the same logically. Dual terminology should be retained.