7.5 Numbers, letters and reference signs 

Numbers, letters and reference signs and any other data given on the sheets of drawings, such as the numbering of figures, pages of the drawing, acceptable text matter, graduations on scales, etc., must be simple and clear, and not used in association with any brackets, inverted commas, circles or outlines whatsoever. Signs such as 6' and 35" are not regarded as including inverted commas and are therefore permitted.

Numbers, letters and reference signs should preferably all be laid out the same way up as the diagram to avoid having to rotate the page.