C

Enamel hatchets use the classic 3-number G.V. Black formula (e.g., 15-8-14); only instruments whose cutting edge is not at a right angle to the blade—such as gingival margin trimmers—carry a 4-number formula, whereas hatchets do not. Spoon excavators may also be in 3-number form, but the enamel hatchet is the prototypical example of a 3-digit instrument. [Smith, 1989, PMID 2628938]