1. Angle, E.H.: Malocclusion of the Teeth, ed. 7, Philadelphia, 1907, S. S. White Co., p. 104-113.
2. Brash, J.C.: The Aetiology of Irregularity and Malocclusion of the Teeth, London, 1929, Dental Board of the United Kingdom, p. 212.
3. Dewey, Martin, and Anderson, G.M.: Practical Orthodontics, ed. 6, St. Louis, 1942, The C. V. Mosby Company , p. 141-3.