the same as the letter height. As a general rule, two thirds of the letter height is a good distance between  lines.  This  spacing  allows  room  for descenders  of  lowercase  letters  and  still  maintains a  clear  space  of  one  third  of  the  letter  height between  the  descenders  and  capital  letters,  or ascenders  of  lowercase  letters  of  the  following line.  Figure  3-56  shows  proper  word  and  line spacing. CENTERING Since the letters of the alphabet vary in width, it is rather difficult to center a line of lettering. Figure   3-57   shows   one   way   of   solving   this problem.  First,  take  a  piece  of  scratch  paper and  letter  in  the  required  line.  Then,  place this  lettering  above  the  area  in  which  your lettering   is   to   go   and   center   it.   Finally,   use the  sample  as  a  guide  to  lettering  the  desired line. Ending a line of lettering at a given point is equally difficult. As in centering, first, letter the line on a piece of scratch paper in order to achieve the proper line length. To  make  lines  of  lettering  come  out  to  a specified length, you must adjust the word and/or letterspacing.  This  adjustment  in  spacing  is  called JUSTIFYING.  A  good  example  of  justifying  is found in the columns of this manual. Notice how all full lines start and stop on the right- and left- hand margins. Usually, you will only find justified lettering  typeset  or  typewritten  by  mechanical means.  However,  if  you  do  have  an  occasion  to justify  your  lettering,  you  should  try  to  keep the  spacing  between  the  words  as  uniform  as possible.   Uneven   spacing   detracts   from   the appearance  of  the  job.  When  it  is  impossible 45.217 Figure 3-57.-Centering with trial spacing paper. to divide the spacing evenly, insert wider spacing at points where one word ends and the next begins with  tall  letters,  like  d, b, and  l. If you use too much space between the words, the  paragraph  will  tend  to  fall  apart  because  it is filled with rivers of white space that will disturb the eye. When  a  line  is  so  short  that  it  calls  for  an undue amount of space between words to lengthen the line, allow more space between the letters in each word. This is known as letterspacing. When words are letterspaced, always allow extra space between words so that they will not seem to run together when they are read. Letterspacing makes short words in titles or headings  appear  longer.  Though  it  frequently improves the appearance of words in caps, letter- spacing  reduces  the  legibility  of  words  in lowercase,  Therefore,  the  process  must  be  used with  caution. MECHANICAL   LETTERING In chapter 2 we discussed pens that are used primarily   for   freehand   lettering.   At   times, however,  you  will  be  tasked  with  preparing drawings, charts, maps, or signs that require the use  of  mechanical  lettering.  When  we  refer  to mechanical lettering, we mean standard uniform characters that are executed with a special pen held in  a  scriber  and  guided  by  a  template.  Mechanical lettering  does  not  normally  require  the  use  of lettering  guidelines.   You   will   use   mechanical lettering  principally  for  title  blocks  and  notes on  drawings,  marginal  data  for  special  maps, briefing  charts,  display  charts,  graphs,  titles  on photographs,  signs,  and  any  other  time  that  clear, legible,   standardized   lettering   is   required.   It should  be  noted  that  freehand  lettering  is  the required   lettering   on   most   of   your   drawings; mechanical  lettering  should  be  confined  to  special uses   similar   to   those   described   above.   The availability  of  mechanical  lettering  devices  should not deter you from the daily practice required to execute   freehand   lettering.   With   continuous practice  you  will  become  proficient  with  both mechanical  and  freehand  lettering. One of the most popular types of mechanical lettering   sets   is   the   LEROY   lettering   set.   A 3-37