Black’s Formula
A standard method used to describe the size, shape and angulation of cavity preparation instruments, created by Dr. Greene Vardiman Black (“the father of modern dentistry”).

Hatchets, chisels and cavity prep instruments have 3 measurements:
(x, y, z), based on a circular 100˚ (centigrade) scale, instead of the typical 360˚ (astronomical) scale.
x= the width of the blade in tenths of millimeters.
y= the length of the blade in whole millimeters.
z = the angle the blade forms with the long axis of the handle.

A fourth number, used on select angle formers and margin trimmers, is placed between the first two numbers ( x, a, y, z ). a= the degree of angle for the cutting edge to the long axis of the handle.