VIEWING PLANE LINES are used to indicate the plane or planes from which a surface or several surfaces are viewed.

CUTTING PLANE LINES are used to indicate a plane or planes in which a sectional view is taken.

Section views are used to give a clearer view of the interior or hidden feature of an object that normally cannot be clearly observed in conventional outside views.

A section view is obtained by cutting away part of an object to show the shape and construction at the cutting plane.

figure 3-38, view A. It shows where the imaginary cut has been made. The single view in figure 3-38, view B, helps you to visualize the cutting plane. The arrows point in the direction in which you are to look at the sectional view.

Figure 3-38, view C, is a front view showing how the object would look if it were cut in half. The orthographic section view of section A-A, figure 3-38, view D, instead of the confusing front view in figure 3-38, view A, is placed on the drawing. Notice how much easier it is to read and understand.

Note that hidden lines behind the plane of projection are omitted in the sectional view. These lines are omitted by general custom, the custom being based on the fact that the elimination of hidden lines is the basic reason for making a sectional view. However, lines that would be visible behind the plane projection must be included in the section view.

Cutting plane lines, together with arrows and letters, make up the cutting plane indications. The arrows at the end of the cutting plane lines are used to indicate the direction in which the sections are viewed. The cutting plane may be a single continuous plane, or it may be offset if the detail can be shown to better advantage. On simple views, the cutting plane should be indicated as shown in figure 3-38, view A. On large, complex views or when the cutting planes are offset, they should be shown as in figure 3-39.

All cutting plane indications should be identified by use of reference letters placed at the point of the arrowheads. Where a change in direction of the cutting plane is not clear, reference letters may also be placed at each change of direction. Where more than one sectional view appears on a drawing, the cutting plane indications should be lettered alphabetically. The letters that are part of the cutting plane indication should always appear as part of the title; for example, SECTION A-A, SECTION B-B, If the single alphabet is exhausted, multiples of letters maybe used. The word SECTION may be abbreviated, if desired. Place the title directly under the section drawing.