This International Standard defines a system of dimensions for panel mounting of equipment. It is applicable to electrical and electrically operated indicating, recording and control instruments. It applies to the following types of instruments with protruding bezels: " instruments with square housing; " instruments with rectangular housing with lateral orientation; " instruments with rectangular housing with upright orientation; " instruments with round housing and square bezel; " instruments with round housing and rectangular bezel. The purpose of this standard is to establish dimensional interchangeability between instruments made by different manufacturers. To fulfil this requirement, a defined set of dimensions has been chosen. Using these dimensions, it should be easy to combine instruments of different sizes on the same panel making good use of the available panel space and to produce a satisfactory layout.