The companion to the RGS 1-2, the “RGS Three Four” preparation calibration instrument completes your calibration set. Made of high-grade stainless steel from precision manufacturing by Hu-Friedy, Dr. Stevenson’s prep evaluation device will easily show you the ideal and maximum allowable occlusal clearance of Onlay, FGC and PFM preparations, and the ideal shoulder dimension on Ceramic restorations of all types. The width of the instrument accurately measures class II amalgam isthmus width, and the length assesses adequate axial wall height. The small end of the instrument measures 1.0 mm in diameter and 3.0 mm in length, and the larger end of the instrument measures 1.5 mm in diameter and 4.0 mm in length. Used by thousands of students and several dental schools, this guide serves to train your eye to within 0.1 mm of ideal in your quest towards preparation mastery.