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CAVITY NOMENCLATURE 25

The base of a cavity, or seat of a filling, is that portion of a cav-
ity situated at right angles to the lines of force to which it is most
likely to be subjected. Generally speaking, this is the gingival or
pulpal wall, or both, where these walls are present, as in a step
cavity.

Fig. 7.—Class Three cavity filled. Fig. 8.—Class Four cavity filled.

Fig. 9.—Class Five cavity filled.
A line angle is formed where two Avails of a cavity meet along
a line and is named by joining the name's of the walls so meeting.
There is hut one exception to this rule. That is where the labial
and lingual walls of a proximal cavity in the incisors and cuspids
meet along a line. By applying the rule this would be called the