lines we will explain in the following paragraphs. In addition, we will explain the use of circles and curved lines at the end of this section. VISIBLE   LINES   represent   visible   edges   or contours of objects. Draw visible lines so that the views  they  outline  stand  out  clearly  on  the  drawing with  a  definite  contrast  between  these  lines  and secondary  lines. HIDDEN LINES consist of short, evenly-spaced dashes and are used to show the hidden features of an object (fig. 2-8). You may vary the lengths of the dashes slightly in relation to the size of the drawing. Always begin and end hidden lines with a dash, in contrast with the visible lines from which they start, except when a dash would form a continuation of a visible line. Join dashes at comers, and start arcs with dashes at tangent points. Omit hidden lines when they are not required for the clarity of the drawing. Although  features  located  behind  transparent materials may be visible, you should treat them as concealed features and show them with hidden lines. CENTER LINES consist of alternating long and short dashes (fig. 2-9). Use them to represent the axis of symmetrical parts and features, bolt circles, and paths of motion. You may vary the long dashes of the center lines in length, depending upon the size of the drawing. Start and end center lines with long dashes and do not let them intersect at the spaces between dashes. Extend them uniformly and distinctly a short distance beyond the object or feature of the drawing unless   a   longer   extension   line   is   required   for Figure  2-8.—Hidden-line  technique. Figure  2-9.—Center-line  technique. dimensioning  or  for  some  other  purpose.  Do  not terminate them at other lines of the drawing, nor extend them through the space between views. 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, draw two thick, short parallel lines at right angles  to  the  center  line.  Use  symmetry  lines  to represent partially drawn views and partial sections of symmetrical parts. You may extend symmetrical view visible and hidden lines past the symmetrical line if it will improve clarity. EXTENSION and DIMENSION LINES show the dimensions of a drawing. We will discuss them later in this chapter. LEADER LINES show the part of a drawing to which a note refers. BREAK LINES shorten the view of long uniform sections or when you need only a partial view. You may  use  these  lines  on  both  detail  and  assembly drawings. Use the straight, thin line with freehand zigzags for long breaks, the thick freehand line for short breaks, and the jagged line for wood parts. You may use the special breaks shown in figure 2-10 for cylindrical and tubular parts and when an end view is not shown; otherwise, use the thick break line. CUTTING PLANE LINES show the location of cutting planes for sectional views. 2-6