vertical line in the left side view represents the back face. Again, there is a line that appears as a  visible  line  in  the  right  side  view  and  as  a hidden line in the left side view. In  the  back  view,  the  block  is  shown  reversed, so  that  the  cutaway  part,  which  appears  to  the right in the front view, appears to the left in the back view. Similarly, the right-hand vertical line in the front view represents the right side of the block, while the right-hand vertical line in the back view represents the left side. As a general observation, it is helpful in view analysis to note that in the top, bottom, and side views,  the  line  that  represents  the  front  face  of the  block  faces  toward  the  front  view  of  the  block. Similarly, in the back view, the line that represents the left side faces toward the left side view of the block. This applies to third-angle projection only. A  point  that  constitutes  a  corner  on  an object  is  sometimes  numbered  for  purposes  of identification  in  various  views  of  the  object.  In a  particular  view  of  an  object,  a  corner  point number may be visible, or it may be hidden, as shown in figure 5-18. In the upper left corner of the  figure,  there  is  an  oblique  projection  of  a block, with a corner numbered 2. You can see that this  corner  is  visible  in  top,  back,  and  left  side views, but hidden in bottom, front, and right side views. The  rule  for  numbering  is  that  for  a  hidden corner  point,  the  number  is  placed  within  the outline,  and  for  a  visible  corner  point,  outside the  outline.  You  can  see  how  the  rule  has  been followed  in  figure  5-18. A  multi-view  projection  should  contain  only as  many  views  as  are  required  to  describe  the object  fully.  If  you  refer  back  to  figure  5-17, you   can   see   at   once   that   the   back   view does  not  convey  any  information  that  is not   available   in   the   front   view;   the   back view   is   therefore   superfluous   and   should   be omitted.  The  same  applies  to  the  bottom  view, which  conveys  no  information  not  available in  the  top  view.   Likewise,  the  left  side  view conveys no information not available in the right side view. You have the choice of omitting either the top or  bottom  view  and  either  the  right  side  or  left side view. One general rule in this instance is that a top view is preferable to a bottom view and a right side view, to a left side view; another rule is that a view with a visible line is preferable to a view with the same line shown as a hidden line. Both rules apply here to eliminate the bottom and the  left  side  views.  All  you  need  here  is  a three-view projection showing the top, front, and right side views. It is often the case that a two-view projection is all that is required. The view at the top of figure 5-19 shows a single-view projection of an object. It  is  obvious  that  a  top  view  of  this  object  tells you  everything  you  need  to  know  except  the thickness; a right side view tells you everything you need to know except the length; and a front view   tells   you   everything   you   need   to   know except the width. All you need to do, then, is to select a particular view and couple it with another view that gives you the dimension that is missing in the first view. There   are   three   possible   two-dimensional projections  of  the  object  shown  in  A,  B,  and  C. In the selection of one of these three, everything else being equal, the balance of the drawing would be  the  deciding  factor.  Either  A  or  B  appears better  balanced  than  C,  and  between  A  and  B, A  would  look  better  on  a  long  oblong  sheet  of paper,  and  B,  better  on  a  shorter  oblong  sheet. The  object  shown  in  figure  5-19  has  a definitely  designated  top  and  front;  it  follows  that the   right   and   left   sides   are   also   definitely designated.  This  is  the  case  with  many  objects; you have no choice, for example, with regard to the  top,  bottom,  front,  and  back  of  a  house. Figure  5-19.-Two-view  multi-view  projections. 5-11