References
Hart, K., Kerslake, D., Brown, M., Ruddock, G., Kuchemann, D. and McCartney, M. (eds) (1981) Children's Understanding of Mathematics 11-16, London, John Murray.
McCague, W. (2003) 'A mathematical look at a medieval cathedral', Math Horizons, April, pp.11-15 and p.31. See also http://www.maa.org [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .
Phillips, E. and Crespo, S. (1996) 'Devopling written communication in mathematics through math penpal letters', For the Learning of Mathematics, vol.16, no.1, pp. 15-22.