15.21.4  CURSOR 

The  cross  hair  cursor  is  used  to  specify the limits of the diamond.
Simply move the cross hairs so they define the lower-left  limits  of  the
diamond  and press the space bar.  Next move the cursor to the upper-right
limits of the diamond an press space again.  If you press a key other than
space,  the  cursor  information is not used.  Alternately you may specify
the upper-left then lower-right extension, or any other  combination  that
defines the diamond.