The use of shading in figures is allowed provided this assists in understanding them and is not so extensive as to impede legibility. Shading may, for instance, be used to indicate the shape of spherical, cylindrical or conical elements. Flat parts may also be lightly shaded. Such shading is allowed in the case of parts shown in perspective but not for cross-sections. Only spaced lines may be used for shading, not fully blacked out areas. These lines must be thin and as few in number as possible and contrast with the rest of the drawings.
PCT AG I 5.158