When the properties of a material are referred to, the relevant units should be specified if quantitative considerations are involved. If this is done by reference to a published standard (e.g. a standard of sieve sizes) and such standard is referred to by a set of initials or similar abbreviation, it should be adequately identified in the description.

Physical values must be expressed in the units recognised in international practice, which is generally in the metric system, using SI units and the other units referred to in Chapter I of the Annex to EEC Directive 80/181/EEC of 20.12.1979, as amended by EEC Directives 85/1/EEC of 18.12.1984, 89/617/EEC of 27.11.1989 and 1999/103/EC of 24.01.2000 (see the Annex to this Chapter). Any values not meeting this requirement must also be expressed in the units recognised in international practice. Values in the inch/pound system, in general, do not meet the criterion "recognised in international practice".

As Rule 49(10) indicates, for mathematical formulae the symbols in general use must be employed. For chemical formulae, the symbols, atomic weights and molecular formulae in general use must be employed.

In general, use should be made of the technical terms, signs and symbols generally accepted in the field in question.