MADSEN, David A. Engineering Drawing and Design. [s.l.]: Thompson Learning, 2001. (Delmar drafting series). Dostupné online. ISBN9780766816343. S. 120..
LEE, John M. Axiomatic Geometry. [s.l.]: American Mathematical Society, 2013. (Pure and Applied Undergraduate Texts). Dostupné online. ISBN9780821884782. S. 134..