The instrument formula of a Gingival margin trimmer is 12 1/2-100-7-14. This instrument cannot be used for:
The gingival margin trimmer is designed to produce a proper bevel on gingival enamel margins of proximoocclusal preparations. When the second number in the formula is 90 to 100, the pair is used on the distal gingival margin. When this number is 85 to 75, the pair is used to bevel the mesial margin. The 100 and 75 pairs are for inlay/onlay preparations with steep gingival bevels. The 90 and 85 pairs are for amalgam preparations with gingival enamel bevels that decline gingivally only slightly. Among other uses for these instruments is the rounding or beveling of the axiopulpal line angle of two-surface preparations.
Ref: Sturdevant's art and science of operative dentistry. 4th edition page 315
Distal gingival preparation
Mesial gingival preparation
Inlay preparation with steep gingival bevel
Onlay preparation with steep gingival bevel