The claims must not, in respect of the technical features of the invention, rely on references to the description or drawings "except where absolutely necessary". In particular they must not normally rely on such references as "as described in part ... of the description", or "as illustrated in Figure 2 of the drawings". The emphatic wording of the excepting clause should be noted. The onus is upon the applicant to show that it is "absolutely necessary" to rely on reference to the description or drawings in appropriate cases (see T 150/82). An example of an allowable exception would be that in which the invention involves some peculiar shape, illustrated in the drawings, but which cannot be readily defined either in words or by a simple mathematical formula. Another special case is that in which the invention relates to chemical products some of whose features can be defined only by means of graphs or diagrams.