"This International Standard specifies the dimensions that are of importance for mechanical interchangeability for ETD-cores made of ferrite@ the essential dimensions of coil formers to be used with them@ and the effective parameter values to be used in calculations involving them. NOTE 1 Whilst this standard mainly applies to ferrite@ its validity for iron powder cores should not be overlooked. NOTE 2 The cores covered by this standard are designed for use in power transformers and chokes operating at high flux density and generally at frequencies higher than those feasible with EC-cores of the same material@ due to a core proportioning more suitable for high-frequency applications. They are generally used in pairs. The use of ""derived"" standards which give more detailed specifications of component parts whilst still permitting compliance with this standard is discussed in Annex A@ which also contains an example of a derived standard for coil formers."