1. 1. CA Harris : The Principles and Practice of Dental Surgery (ed 8 ). Philadelphia, PA, Lindsay & Blakiston, 1863 , p685 .
2. 2. J Richardson : A Practical Treatise on Mechanical Dentistry (ed 1 ). Philadelphia, PA, Lindsay & Blakiston, 1860 , p275 .
3. 3. GH Wilson : A Manual of Dental Prosthetics (ed 2 ). Philadelphia, PA, Lea & Febiger, 1914 , pp159 -160 .
4. The Glossary of Prosthodontic Terms Seventh Edition (GPT-7)