In orthographic projection it is sometimes required that the shape of internal, hidden parts to be shown. This is usually achieved by drawing a sectional view of the object being drawn. To draw a sectional view, one has to imagine the object being cut through a plane and only drawing the part that is further away from the observer. The cutting plane is usually indicated in an adjacent view. Looking at the example presented, note that in the plan the horizontal centre line is actually the cutting plane. This is indicated by the arrows, the darker lines at the ends of the line and by the labels AA. In this case, the sectional view has been labelled AA for easy reference; this is normal practice.
Note also that the parts of the material cut by the plane are shown shaded by thin lines at 45 degrees and equally spaced from each other. Those are referred to as the section lines or hatching lines. An exception to this rule is when the plane goes through a web or a rib and cuts across it. Also, hidden lines are not to be shown in sectional views, but centre lines should be. Most students forget to draw centre lines, especially if the associated hole is not shown in the view.
The following are some notes and short exercises including the model answers.