This part of IEC 917 may be applied to all fields of electronics in which equipment and installations with a modular design are used. The object of this part of IEC 917 is to define a sectional specification for the mechanical structure of a 25 mm equipment practice to provide for dimensional compatibility at mechanical interfaces with related engineering applications, e.g. plug-in units, printed boards, components, instrumentation, furniture, rooms, buildings, etc. It specifies the co-ordination dimensions and pitches for cabinets, racks, subracks and chassis of all types used for electronic equipment in, for example industrial electronics,information technology, measurement and control systems and communications. The chosen co-ordination dimensions for the mechanical structure for heights, widths and depths, lie within a homogeneous, metric modular three-dimensional grid as specified in IEC 917. In this pa rt of IEC 917, the mounting pitches 25 mm and 2,5 mm have been used as a base to determine the interface co-ordination dimensions for the electronic equipment practice. This creates the optimum conditions for the application of computer-aided methods throughout design and manufacture such as, for instance, computer-aided planning,development, manufacturing, testing and installation. The cabinets and racks in this part of IEC 917 represent a specific structural form which takes into consideration for instance the installation of subracks and chassis. The determined interface co-ordination dimensions for the overall outside and aperture dimensions of subracks provide space for independently created plug-in units to be installed. These plug-in unit dimensions shall follow the rules of IEC 917-0 and IEC 917 and are to be determined in detail specifications. The connector system used (0,5; 1,0; 2,0; 2,5 •mm, etc. connector grid) and the special application requirements are to be determined in associated detail specifications. Further standards may be used in conjunction with this pa rt of IEC 917: IEC 297-1: 1986, Dimensions of mechanical structures of the 482,6 mm (19 inch) series – Part 1: Panels and racks. IEC 297-2: 1982, Dimensions of mechanical structures of the 482,6 mm (19 inch) series – Part 2: Cabinets and pitches of rack structures.