In exceptional cases, chemical or mathematical formulae may be written by hand or drawn if necessary, but it is recommended that appropriate aids such as stencils or transfers be used. For practical reasons, formulae may be grouped together on one or more sheets annexed to the description and paginated with it. It is recommended in such cases that each formula be designated by a reference sign and the description should contain references to these formulae whenever necessary.
The chemical or mathematical formulae must employ symbols in general use and must be drawn in such a way that they are completely unambiguous. Figures, letters and signs which are not typed must be legible and identical in form in the various formulae, irrespective of the document in which they appear.
All mathematical symbols used in a formula which appears in a description, in an annex or on sheets of drawings must be explained in the description, unless their significance is clear from the context. In any case, the mathematical symbols used may be collated in a list.