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60 OPERATIVE DENTISTRY

both buccal and lingual embrasures until the excursions of food
through these embrasures will sweep the margins of the completed
filling for its entire length. This extension will result in carrying
the outline out sufficiently that it can be seen to pass under the gum
in full view.
A Good Rule to Follow is to cut sufficiently that a chisel one
millimeter in width will pass easily from the embrasures to the
open cavity when dragging the cutting edge lightly over the free
margin of the gum. This is stated as a general rule only, there
being circumstances which would permit falling short of this amount
of space and yet there are cases which demand a greater amount
of cutting to fully meet the requirements of extension for preven-
tion, due to oral conditions and dental irregularities.

A B
l"ig. -One of the few cases in which the step may be omitted in Class Two cavities.

Extensions Gingivally. The cavity outline should be carried sub-
gingivally in extension for prevention w^hen from other reasons
that part of the outline approaches to within one millimeter of the
gum line. The application of this rule Avill invariably cause the
outline to go beneath the gum in case the gum is in or resumes its
normal position.
If there is reason to believe that it will return to its normal
position this fact should be considered. In cases of permanent re-
cession it is better to stop the cavity outline midway from contact
to gum line.
Care at Axio-Gingival Angles. Tlie buccal and lingual portions
of the outline should be carried directly gingivally and be made