The term zone has been used both descriptively and interpretively in stratigraphy. Descriptively, zone means a body of rock that bears fossils; interpretively, zone means a body of rock, the fossiliferous content of which is interpreted to be the fossilized remains of either (1) an ecologic formation (community type), (2) a community, (3) a congregation of a time-stratigraphic unit, (4) either the biogeographic provincial or the total stratigraphic range of a taxon, or (5) only the flowering (flood or peak) of a taxon. Most of the interpretive uses of zone have not been discussed in the American Code of Stratigraphic Nomenclature, although they are cited widely and confused commonly with the descriptive uses. They are discussed with emphasis on the time-stratigraphic kind of zone and with brief reference to the descriptive uses of zone.

The several uses of zone commonly have been confused in studies of the Ordovician; hence, they have given rise to differences in opinion concerning Ordovician zones and their correlation. The Ordovician graptolite zones are time-stratigraphic. They have been established on diagnostic congregations of species which have been worked out from the analysis of overlapping vertical stratigraphic ranges of many species. They are limited in their applicability to a single zoogeographic Province. The boundaries between them are based on evolutionary development in graptolite phyletic lineages in the Province as well as on sudden appearances of invaders from another Province.

Both North American and British Ordovician graptolites developed in separate zoogeographic Provinces for at least the early half of the period. Not until the Middle Ordovician was there a large-scale invasion of the American Province by members of several lineages established in the British Province. This invasion of the American Province by British-origin stocks permits close correlation of the graptolite zonal sequences in each Province from the time it occurred. Before that time, the general grade of evolutionary development represented by the plexus of didymograptids, tetragraptids, and phyllograptids common in each Province is considered to be essentially correlative. The graptolites with biserial scandent rhabdosome form are considered to have developed in the British provincial area and then to have invaded the American Province.