Layers and all their properties, including classifications, can be duplicated using ->. Duplicating a layer is useful if the user wishes to model a layer in several different ways. Even though the layers overlap, by carefully selecting the shape properties it is possible to display several pieces of information at once. For example, one copy of a roads layer may be classified on a length property and another copy may be classified on a type property. If the length property was expressed with color and the type property expressed with line thickness then it would be possible to view both classifications by placing the type property copy over the length property copy.