42 Chapter 3
The long dashes of center lines may vary in length, depending upon the size of the drawing. Center
lines should begin and end with long dashes, and they should not intersect at the spaces between the
dashes. They should extend uniformly and distinctly a short distance beyond the object or feature of the
drawing unless a longer extension line is required for dimensioning or for some other purpose. Very short
center lines may be unbroken if there is no confusion with other lines.
Symmetry lines are center lines used as axes of symmetry for partial views. To identify the line of
symmetry, two thick, short parallel lines are drawn at right angles to the center line. Symmetry lines are
Figure 3.4A Sectional lines add emphasis to the walls in this illustration.