For strength of amalgam, the axial depth is a minimum of 1.0 mm measured at the gingival wall. For preservation of tooth structure, the axial wall is prepared parallel to the external tooth surface which is flat or slightly convex, never concave.
Buccal and lingual walls are prepared perpendicular to the tangent drawn at the predetermined clearance of 0.5 mm from the adjacent tooth. This insures both fully supported enamel rods and butt-joint design for edge strength of amalgam.
