Usually the first lines that you will use on a drawing are construction lines. These are the same lines that you used to lay out your drafting sheet. They will also be used to lay out the rest of your drawing. Line weight for construction lines is not important since they will not appear on your finished drawing. They should be heavy enough to see, but light enough to erase easily. A 4H to 6H pencil with a sharp, conical point should be used. With the exception of light lettering guidelines, all construction lines must be erased or darkened before a drawing is reproduced.

Center lines are used to indicate the center of a circle, arc, or any symmetrical object. (See fig. 3-24.) Center lines are composed of long and short dashes, alternately and evenly spaced, with a long dash at each end. They should extend at least one-fourth in. outside the object. At intersecting points, center lines should be drawn as short dashes.

A very short center line may be drawn as a single dash if there is no possibility of confusing it with other lines. Center lines may also be used to indicate the travel of a moving center, as shown in figure 3-24.

Figure 3-24.-Use of center lines.

The visible edge lines of the view are drawn as solid, thick lines. These include not only the outlines of the view, but lines defining edges that are visible within the view. (See fig. 3-25.)