The drawings must be executed in durable, black, uniformly thick and well-defined lines and strokes. See GL/PCT-EPO A-V, 1.2, in respect of colour drawings, where these are exceptionally submitted. In all cases, the thickness of the lines and strokes must take into account the scale, nature, execution and perfect legibility of the drawing and of the reproductions. All lines in the drawings must, ordinarily, be drawn with the aid of a drafting instrument, except for those which by their nature do not permit the use of such instruments, for example irregular diagrams, ornamental structures and curved reference lines.[R. 11.13(a); 11.13(f); ]