Numbers, letters, reference signs and any other data given on the sheets of drawings, such as the numbering of figures and of the sheets, 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 indicating minutes, seconds or degrees are permitted. Numbers, letters and reference signs should be laid out in the same direction as the diagram so as to avoid having to rotate the sheet. They should not be placed in the closed and complex parts of the drawings so as to interfere with a thorough comprehension of the drawings, and therefore should rarely cross or mingle with the lines. As a general rule, numbers, letters and reference signs should be placed as close as possible to the part in question.
PCT AG I 5.152