Chronostratigraphy is concerned with age of strata and time relations; it is an attempt to organize the sequence of strata into units that relate to time, determine local time relations, and correlate globallyand thus establish a standard global chronostratigraphic scale. The nomeclature involves chronzone, stage, and series.
Chronostratigraphic units are bodies of rock that formed during a specific time interval; the chronostratigraphic unit represents all rock on globe that formed during the time interval. The boundaries of chronostratigraphic units are isochronous surfaces or time lines. The rank of chronostratigraphic units is based on time not thickness.
Chronostratigraphic horizon is an isochronous stratigraphic surface; theoretically it has no thickness. In the real world we use very thin distinctive intervals; called markers, marker horizon, marker bed, key bed, time surface, datum, *real world examples
Geochronologic units are framiliar names found on thegeologic time scale; they relate time only and provide an arrangement of time units of worldwide scope for reference dates. The precision or effectiveness of geochronologic units decreases with size - due to local variation; look at history of the Pliocene.
Geochronometric units are an abstraction. They have no type section and are new part of 1983 code based on an arbitrary agreement to divide time based on years
Eonothem/eon typically represent 500-1,000 ma. The Precambrian eons comprise 85% of all geologic time, lack real precise boundaries and are based on large scale tectonic movement.
Erathem/era represent hundreds of millions of years. Several adjacent system comprise an erathem. Phanerozoic eras and erathems are named for major changes in life.
System/period typically represent 35-70 ma with an average near 50-60 ma. The Phanerozoic names are accepted and will not change. The Precambrian nomenclature varies: DNAG divided Proterozoic into Early, Middle & Late others subdivided either I,II, III or X ,Y, Z
Series/epoch typically represent 15 ma. There are 2-6 series in system with 3 in most. These units are larger than a stage/age but smaller then system/period.
Stage/age have an average durationof 3-10 ma. The stage is basic chronostratigraphic working unit in that it is of a practical size that allows correlation between regions. Type sections of these units are variable in thickness.
Chronozone/chron typically don't last very long and are part of stage or age. These are generally the smallest units in respective classification scheme and are nonhierarchial. The relationship of chronozone to biozone is shown below.

Chronostratigraphic nomeclature is part of a sequence so use lower, middle, andupper.
eonothem - erathem - system* - series* - stage* - chronozone
Geochronologic nomeclature is time based so use early, middle, and late.
eon - era* - period* - epoch* - age* - chron *most commonly used
Cambrian, Ordovician, Devonian, Triassic, Jurassic periods have epochs named early, middle and late. The Silurian, Carboniferous, Permian, Cretaceous periods are divided into early and late epochs.
Cambrian, Devonian, Triassic & Jurassic series are lower, middle, upper
Fossil based chronostratigraphic classification is based on the appearance/disappearance of a taxon. Naming procedure are similar to other varieites of stratigraphic units and should include adequate stratotype with units that fill the type section with no gaps or overlaps. This would be easy to do if 1. there was one complete section for all geologic time or 2. time correlation were perfect allowing positive identification of horizons between regions.
Ideally one should define a unit by boundary stratotypes not space in section. This eliminates gaps and overlaps. It is desirable to have same boundary for series, stage, chronozone, system etc. thus providing chronostratigraphic classification with no breaks.