ISO 868:2003 specifies a method for the determination of the indentation hardness of plastics and ebonite by means of durometers of two types: type A is used for softer materials and type D for harder materials. The method permits measurement either of the initial indentation or of the indentation after a specified period of time, or both. This method is an empirical method intended primarily for control purposes. No simple relationship exists between indentation hardness determined by this method and any fundamental property of the material tested. For specification purposes, it is recommended that ISO 48, Rubber, vulcanized or thermoplastic -- Determination of hardness (hardness between 10 IRHD and 100 IRHD), be used for the softer materials.