Stratigraphic data constitute an open system of numbers that can be converted to a closed system by computing percentages and ratios. Percentage and ratio maps play an important part in facies studies, and it is appropriate to consider the changes that occur in maps when the data are closed. In 2- and 3-component systems the statistical correlations that arise from closing the data can be exactly predicted, but the restraints are less rigid for systems with 4 or more components. The behavior of 2- and 3-component systems is illustrated with synthetic data, followed by an example of a 4-component stratigraphic unit from the Upper Permian of western Kansas and eastern Colorado. Analysis of the map data before the maps are actually drawn shows that some general predictions can be made regarding the particular maps that may give most information within the context of a specific problem. This theme is developed with the Permian example, and leads to a tentative flow chart of map selection based on computation of summary statistics, of all linear correlations, and of trend surfaces and their deviations.