The Common Graphics subsystem uses a pool dictionary called CgConstants. This dictionary provides pool variables for constant values that are used by the subsystem. For example, pool variables such as GCForeground and GCLineWidth are used to indicate which graphics context attributes to change in the createGC:values: and changeGC:values: methods. Other pool variables, such as FillStippled and LineSolid, are used to represent various GC attribute values. Pool variable names should be used rather than directly using their constant values.