1. N.W. Chilton(1967 ) Design and Analysis in Dental and Oral Research , p.69 . Lippincott Company, Philadelphia.
2. Preparation areas resisting displacement of artificial crowns
3. J.F. Johnston, R.W. Phillips, and R.W. Dykema(1971 ) Modern Practice in Crown and Bridge Prosthodontics , 3rd edn , p.84 . W.B. Saunders Company, Philadelphia, London, Toronto.