Figure 7 consists of a schematic drawing and a photograph. Both images are representations of the same ball point pen with the top removed and placed with it. The photograph of the pen is there to help the viewer interpret the drawing so we should concentrate on the drawing. The drawing of the pen is called a longitudinal section, which is a form of engineering drawing. A longitudinal section is a bit like cutting a baguette in half lengthways – each half reveals what it is like inside the loaf. Imagine the pen cut in half along its longitudinal axis so that you can see the insides which are also cut in half. Various components of the pen can now be identified and depicted. In this case, the pen is shown as having 6 components – an end plug, an ink tube filled with ink, an outer barrel, a ball, a metal piece designed to hold the ball and a cap. In this case, the pen is shown in a partly assembled state. The ink tube is in the barrel but the end plug, metal piece, ball and cap are shown separately. All these components are laid out in a straight line as if the pen had been literally pulled apart. Since the pen has a transparent outer barrel, all the parts with the exception of the ball can be seen in the photograph. Even the ball could be seen with close physical inspection. The section drawing comes into its own when the internal components and in turn their internal features cannot be seen or felt. Engineers also use their drawings to convey sizes by using a system of arrows and lines. Lines are used to identify the positions of parts of the object and a line with an arrow head at each end, which is called a double headed arrow, is used to denote the distance between those lines. Next to the line is written the dimension. In this drawing two dimensions are identified. The length of the outer barrel is 125mm and the length of the metal piece is 20 mm.