Original: In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full fledged geologic period between the Mississippian and the Permian.
Simplified: The Pennsylvanian is the Upper Carboniferous period in the strata of North America. It comes after (above) the Mississippian, and before the Permian period.