Which of the following is not true regarding the second number of the instrument formula?: (A) Indicates the primary cutting edge angle, (B) The angle is expressed as a percent of 360 degrees, (C) This number is always 50 or less, (D) It can be omitted to give 3 number code
Answer is C. The second number of a four-number code indicates the primary cutting edge angle, measured from a line parallel to the long axis of the instrument handle in clockwise centigrades.
The angle is expressed as a percent of 360 degrees (e.g., 85 = 85% x 360 degrees = 306 degrees).
The instrument is positioned, so that this number always exceeds 50.
If the edge is locally perpendicular to the blade, then this number is normally omitted, resulting in a three-number code.

Ref: Sturdevant's art and science of operative dentistry, 4th edition page 311