This International Standard specifies the functional connecting dimensions of the front faces of spindle holders for machining centres and of their various machining and measuring accessories to ensure that interchangeability, fixing by means of automatic tool changing devices and correct machine operation of these accessories is possible. The main accessories are a) electrically operated accessories; b) coolant feed accessories, and c) mechanical indexing accessories, i.e. 1) angle heads, 2) drilling-tapping heads, 3) speed-increasing gears, etc.