In Fig. 108 there is represented a rectangular prism or block, whose length is twice the width. The elevation shows its height. As the prism is placed at an angle, three of the vertical edges will be visible, and the fourth, invisible.

Fig. 108. Proper Representation of Rectangular Prism.

In mechanical drawing the edges winch in projection form a part of the outline or contour of the figure must always be visible, hence are always drawn as full lines, while the lines or edges which are invisible are drawn dotted. The plan shows what lines are visible in elevation, and the elevation determines what are visible in plan. In Fig. 108, the plan shows that the dotted edge A B is the back edge, and in Fig. 100, the elevation shows that the dotted edge C D is the lower edge of the triangular prism. In general, if in elevation an edge projected within the figure is a back edge, it must be dotted, and in plan if an edge projected within the outline is a lower edge it is dotted.

Fig. 109. Representation of Triangular Prism.

Fig. 110 is a circular cylinder with its length vertical and with a hole part way through as shown in the elevation. Fig. 1ll is the plan, elevation, and end view of a triangular prism with a square hole from end to end. The plan and elevation alone would be insufficient to determine positively the shape of the hole, but the end view shows at a glance that it is square.

Fig. 110 Projection of Circular Cylinder.

Fig. 111. Plan, Elevation, and End View of Triangular Prism.