A common and precise language of geologic time is required to discuss and unravel Earth's history. One of the main goals of the International Commission on Stratigraphy has been to unite the individual regional scales by reaching agreement on a standardized nomenclature and hierarchy for stages defined by precise Global Boundary Stratotype Sections and Points (GSSPs).
This task proved to be more challenging than envisioned when the GSSP effort began in the 1980s; but it is finally nearing completion. The choice of the primary criteria for an international stage boundary can be a contentious issue, especially when competing regional systems or vague historical precedents are involved. Preference for stratigraphic priority is laudable when selecting GSSPs, but subsidiary to scientific and practical merit if the historical versions are unable to provide useful global correlations. In particular, the Cambrian and the Ordovician subcommissions developed a global suite of stages after the initial step of achieving correlation among regions, rather than trying to fit global Earth history into the limitations of pre-existing regional American, British, Chinese or Australian stages. However, regional stages will always remain very useful, and the ICS has compiled inter-regional correlation charts to place these geologic successions into the global framework.
Approximately one-quarter of the ca. 100 geologic stages still await an international definition based on precise GSSPs. Those that remain undefined have either encountered unforeseen problems in recognizing a useful correlation horizon for global usage (e.g., base of Cretaceous System), a desire to achieve calibration to other high-resolution scales (e.g., base of Langhian stage in Miocene awaiting astronomical tuning), inability to reach majority agreement, or another difficulty. These aspects should be solved within the next two years, and geologists will at last achieve a full suite of global stages.