This section of IEC 917-2 is a detail specification which is to be used in whole or in part in all fields of electronics where devices and systems are designed according to the sectional specification IEC 917-2. The purpose of this publication is the specification of dimensions which will ensure mechanical interchangeability of subracks, chassis, plug-in units, backplanes and front panels. The sectional specification IEC 917-2 has embodied within it a number of subrack systems based on 25 mm three-dimensional modularity and the binary additive principle for subrack heights. The dimensions in the tables of this detail specification are based on one of the subrack systems taken from the sectional specification IEC 917-2. However, all subrack systems within the sectional specification are valid in this detail specification, provided the relationship between the dimensions is maintained, i.e. the dimensional difference between the co-ordination dimensions HS, Ws and DS and dimensions HS0 , etc.; WS0 , etc., and Dso,etc., shown in tables 1, 2 and 3 are to be maintained. The subrack dimensions are related to certain cabinet/rack dimensions, see IEC 917-2-1. The front panel dimensions are related to the mounting dimensions of certain cabinets and racks, see IEC 917-2-1. Plug-in units are mounted in subracks. They serve as supports for electrical, electronic and mechanical components. The electrical connection between plug-in units and subracks is based on the use of connectors. All dimensions are given in millimetres and the drawings are first angle projections.