This International Standard specifies the dimensions that are of impo rtance for mechanical interchangeability for a preferred range of PM-cores made of magnetic oxides, the main dimensions for coil formers to be used with these cores and the locations of their pins on a modular printed wiring grid in relation to the base outlines of cores. It also specifies the effective parameter values to be used in calculations involving these cores. The use of derived standards which give a more detailed specification of component parts whilst still permitting compliance with this standard is discussed in annex A. PM-cores are so called because they are derived from pot-cores and have cut outs permitting the use of coil formers containing terminal pins for mounting on a modular printed wiring board. They are mainly intended for use in power applications such as switched mode power supplies (SMPS), pulse power transformers and broadband transformers. For these applications the pot-core shape gives the advantage, compared with other shapes, of a relatively large cross-section with correspondingly high magnetic flux, which in turn permits windings of fewer turns for a given power, with the associated benefits of low leakage inductance and self-capacitance. These cores also provide good shielding, easy mounting and enable precise air-gaps to be ground. PM-cores are also of interest for new SMPS techniques involving higher powers at frequencies greater than 50 kHz, since, because they possess relatively small openings, the stray field losses which increase with frequency are much smaller than with any E-core type. The present range of PM-cores contains five sizes, four of which are suitable for mounting directly on a printed wiring board with a simple U-bolt mounting. The locations of the mounting pins for these cores and their numbering are specified: they are at spacings of two modules and their possible number varies from 14 to 20. The largest core is too heavy for mounting on a printed wiring board, and neither the number or location of its terminal pins, which are usually located within the cut-out of the core, are specified.