This part of ISO 7750 specifies the essential dimemsional characteristics necessary for interchangeability of filter elements for air filters generally used in commercial vehicles. Two element types, C and D, are specified; which in their design and which require different installation conditions (see annex A and B). Types C and D are not interchangeable. The selected sizes specified in tables 1 and 2 are based upon popular elements currently on the market. This part of ISO 7750 may be used for other air filter applications where no specific standards exist.