Figure 17 consists of two diagrams; one represents a unit of area and the other represents a unit of volume. The unit of area is represented by a square. Each side is labelled as being 1 metre long and the space bounded by the square is labelled as being 1 metre squared. The unit of volume is represented by a cube. The cube is drawn using a technique of drawing called oblique projection. Whenever a cube is viewed, only 3 of the 6 faces will be visible and their shape will always be distorted by the effects of perspective. Perspective drawing is very difficult and time consuming and so engineers utilize other methods of drawing a solid object. Oblique projection is perhaps the easiest as one face of the object is drawn using its true geometric shape. In this case, one face of the cube is drawn as a square. The right hand surface and the top surface are also visible. Consider the top surface. The edge shared with the front surface is horizontal, and the edge shared with the rear hidden surface is also horizontal. The side edges are drawn at an angle which in this case is about 38 degrees. The sides are drawn a little shorter. The top surface is therefore drawn as a parallelogram. Consider the right hand side surface. The edge shared with the front surface is vertical, and the edge shared with the rear hidden surface is also vertical. The top edge is shared with the top surface and is at the same angle of about 38 degrees. The bottom edge is shared with the hidden bottom surface of the cube and is parallel with the top edge. The side surface is therefore drawn as a parallelogram. A cube has 12 edges but only 9 edges are visible. Although a square has 4 edges and three square surfaces are visible, the side and top surfaces each share an edge with the front surface and share another edge with each other. The cube has another 3 edges and 3 surfaces that are not visible to the viewer. The hidden edges are shown using another technique of drawing – they are drawn with dashed lines. One of these hidden edges is vertical, one is horizontal and the other is at the angle of about 38 degrees. A cube has 8 corners but only 7 are visible. The three hidden lines join at that hidden corner and as a result the three hidden surfaces are revealed by the geometry of the lines. Each side of the cube is labelled as 1 metre long and the space inside the cube, which represents the volume of the cube, is labelled as 1 metre cubed.