As in lithostratigraphy and biostratigraphy, time-stratigraphic units may be classed on the basis of their vertical, lateral, and vertico-lateral contiguity.

Those space-time units defined by their position in vertical succession are of two kinds: (1) units of variable time value without specific time limitations, including holostrome, hiatus, lacuna, and erosional vacuity; and (2) units of constant maximum time limits, including (a) units of constant time value such as era, period, epoch, and age, and (b) units of variable time value within specified maximum time limits such as system, series, and stage.

Holostrome and hiatus are analogous to formation in lithostratigraphy and zonule in biostratigraphy. Neither era-period-epoch-age, nor system-series-stage, have strict analogies in lithostratigraphy or biostratigraphy.

Laterally contiguous time-stratigraphic facies based on comparative ratios of two or more of the afore-mentioned time-variable units exist in concept, and are amenable to cartographic portrayal if desired. Such facies in time-stratigraphy are analogous with lithofacies and biofacies.

Vertico-laterally contiguous (mutually intertongued) combined holostromes comprise depositional holosomes, and combined hiatuses comprise hiatal holosomes. These are the time-stratigraphic analogies of lithosome and biosome. Their primary function is to serve as a basis for envisaging the complete regional or interregional, continuous, and differential migration of depositional and nondepositional environments in space and time.