Cavosurface Margin

It is the junction of the prepared and the unprepared surface of the tooth. Because of the property of the amalgam being brittle when the bulk of material is thin, it should be placed on a tooth with a cavosurface margin of 90 degrees. Also, this angle should be present when making proximal boxes. An exit angle of 90 degrees on the proximal cavosurface margins will result to an adequate bulk of amalgam, preservation of tooth structure, and less creation of unsupported enamel.