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should be differentiated from hidden, extension, dimension,  or  center  lines. CONSTRUCTION  LINES 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 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. VISIBLE  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.) HIDDEN  LINES Hidden edge lines are drawn with short dashes and  are  used  to  show  hidden  features  of  an object.  A  hidden  line  should  begin  with  a  dash in  contact  with  the  line  from  which  it  starts, except  when  it  is  the  continuation  of  an  unbroken line.  (See  fig.  3-26.) To  prevent  confusion  in  the  interpretation of  hidden  edge  lines,  you  must  apply  certain  stan- dard   techniques   in   drawing   these   lines.   A hidden edge line that is supposed to join a visible or another hidden line must actually contact the line, as shown in the upper views of figure 3-27; Figure 3-25.-Use of visible edge lines. Figure 3-26.-Use of hidden edge lines. Figure 3-24.-Use of center lines. Figure  3-27.-Correct  and  incorrect  procedures  for  draw- ing adjoining bidden lines. 3-21

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