A system in stratigraphy is a sequence of strata (rock layers) that were laid down together within the same corresponding geological period.
Systems are unrelated to lithostratigraphy, which subdivides rock layers on their lithology.
The systems of the Phanerozoic were defined during the 19th century, beginning with the Cretaceous (by Belgian geologist Jean d'Omalius d'Halloy in the Paris Basin) and the Carboniferous (by British geologists William Conybeare and William Phillips in 1822).
The one-time system names of Paleocene, Eocene, Oligocene, Miocene and Pliocene are now series within the Paleogene and Neogene.
Another recent development is the official division of the Proterozoic into systems, which was decided in 2004.