DispCmd is a base class for a large number of objects which are used to create the display lists in Displayable objects. Each subclass represents a particular drawing primitive, such as plotting a line, changing the current color, or applying a rotation. These objects take the data required for the particular primitive they represent (for example, the endpoints of a line, or the index of a color) and append this data to the end of the display list for a given Displayable. Routines are available to permit a DispCmd to be used more than once, and also to have the DispCmd remember the location where the data was most recently appended so that new data may be copied over existing data in a display list.