1. French, M.J., Engineering Design: the conceptual stage, Heinemann Educational Books, London 1971, p. 2, pp. 12–40.
2. Pahl, G. and Beitz, W., Konstruktionslehre, Springer-Verlag, Berlin, 1977, Fig. 3.3.
3. BS7000, Guide to Managing Product Design, British Standards Institution, London, 1989, p. 17.
4. Morrison, D., Engineering Design, McGraw-Hill, London, 1971, pp. 35–38.
5. Hume, K.J., Engineering Metrology (3rd ed) Macdonald, London, 1970, pp. 291–8.