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curve that should be concave mesio-distally. The labial enamel wall may
extend farther laterally than the pulpal, and its margin should form a
curve. The lingual wall should be a strong plate of enamel and should
run at right angle to the pulpal wall (Fig. 13). The pulpal wall should
extend from the gingival to the incisal wall in such relation that no
undercuts will be formed. The incisal wall .should form an obtuse angle
with pulpal wall, and it should unite with the lingual wall so that the
same kind of an aiigle will be formed. This permits the gingival and
incisal walls to diverge slightly toward the labial surface.

Fig. 16.

Fig. 14. Fig. 15.

In cavities involving the approximal and lingual
surfaces, this method of cavity preparation should be
eaoities Involving
JIPDroximal and used in those cases in which the labial wall of
Cingual Surfaces. enamel is retained to support the incisal wall. In
many cases the incisal wall of enamel can be re-
tained with safetv, where it receives the support of the labial plate of
enamel. On the other hand, if the labial plate were involved it would
also require the incisal angle to be involved.
The gingival wall should extend rootward far enough to insure a
strong mass of porcelain on the lingual surface, and its margin should
form a curve (Fig. 14). A groove may be formed, from one to three
millimeters deep, which should unite with the labial and lingual walls
in a curve so that no undercuts are formed with these walls, because
such undercuts would interfere with the withdrawal of the matrix. The

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