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128 PREPARATION OF CAVITIES

Rule 3.—In proximo-occlusal cavities in other cases than
the above, use the double, compoxind or auxiliary mortise
form (step form) (Fig. 149, 150 and 151).

Fig. 145. Fig. 146.
Fig. 145.—Application of the simple dovetail mortise form to a disto-occlusal
cavity in an upper molar, where the occlusal surface presents good union of enamel
plates and is free from decay.
Fig. 146.—Cavity similar to that shown in Fig. 145.

Fig. 147. Fig. 148.
Fig. 147.—Application of the compound mortise form to a mesio-occlusal cavity
on an upper bicuspid.
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Fig. 148.—Another view of the cavit}' shown in Fig. 147.
Rule 4.—In proximo-incisal cavities, where the loss of the
mesio- or disto-incisal angle is not extensive and where the

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