Agreed specifications which define the common interfaces between computer systems or subsystems. Standards, if they are generally observed by manufacturers, promote the interchangeability of computer equipment. When standards promote the portability of programs, this is called device independence.
In computer graphics, standards have evolved slowly and painfully. Most of them have at one time needed to have the qualifying adjective "de facto" or "proposed" attached to them, indicating that they are not absolute "standards" in the full sense of the word. However, the standards that now exist, or are in the course of being ratified at a national or international level, are helping to rationalize the interfacing of graphics systems.
The ISO (International Standards Organization) Basic Reference Model of computer architecture defines a system in terms of layers. Thus, the best way to consider current graphics standards is to categorize them according to their individual levels in the layered architecture of a computer system (see figure).
The top level is occupied by IGES (Initial Graphics Exchange System). This standard governs the exchange of CAD objects between application specific databases, allowing an object created on one system to be transferred to another (otherwise incompatible) CAD system for processing.