Abstract Evaluation of time and correlation work within the Devonian suffer greatly from the lack of internationally agreed upon boundaries. The only boundary of the whole system which has been internationally agreed upon is its lower boundaryan excellent example of a precisely defined boundary based on biochronologic evidence. The IUGS Subcommission on the Silurian-Devonian Boundary which defined it suggested this boundary (the stratotype of which is in Czechslovakia) to be within a time span of between 100,000 to 300,000 years, as estimated from biologic evidence. There is no absolute dating in the Devonian that could compete with such a highly precise biochronologic date. The range of error of any published absolute data is many times larger than the time span represented by this lower boundary of the Devonian.