by a letter and a brief description of the revision. A revised drawing is shown by the addition of a letter to the original number, as in figure 1-1, view A. When the print is revised, the letter A in the revision block is replaced by the letter B and so forth. Drawing Number Each blueprint has a drawing number (fig. 1-1, views A and B), which appears in a block in the lower right corner of the title block. The drawing number can be shown in other places, for example, near the top border line in the upper corner, or on the reverse side at the other end so it will be visible when the drawing is rolled. On blueprints with more than one sheet, the information in the number block shows the sheet number and the number of sheets in the series. For example, note that the title blocks shown in figure 1-1, show sheet 1 of 1. Reference Number Reference numbers that appear in the title block refer to numbers of other blueprints. A dash and a number show that more than one detail is shown on a drawing. When two parts are shown in one detail drawing, the print will have the drawing number plus a dash and an individual number. An example is the number 811709-1 in the lower right corner of figure 1-2. In addition to appearing in the title block, the dash and number may appear on the face of the drawings near the parts they identify. Some commercial prints use a leader line to show the drawing and dash number of the part. Others use a circle 3/8 inch in diameter around the dash number, and carry a leader line to the part. A dash and number identify changed or improved parts and right-hand and left-hand parts. Many aircraft parts on the left-hand side of an aircraft are mirror images of the corresponding parts on the right-hand side.  The  left-hand  part  is  usually  shown  in  the drawing. On some prints you may see a notation above the title block such as “159674 LH shown; 159674-1 RH opposite.” Both parts carry the same number. LH means left hand, and RH means right hand. Some companies use odd numbers for right-hand parts and even numbers for left-hand parts. 1-4 Zone Number Zone  numbers  serve  the  same  purpose  as  the numbers and letters printed on borders of maps to help you locate a particular point or part. To find a point or part, you should mentally draw horizontal and vertical lines from these letters and numerals. These lines will intersect at the point or part you are looking for. You will use practically the same system to help you  locate  parts,  sections,  and  views  on  large blueprinted  objects  (for  example,  assembly  drawings of aircraft). Parts numbered in the title block are found by looking up the numbers in squares along the lower border. Read zone numbers from right to left. Scale Block The scale block in the title block of the blueprint shows  the  size  of  the  drawing  compared  with the actual size of the part. The scale may be shown as 1 = 2 , 1 = 12 , 1/2 = 1´, and so forth. It also may be shown as full size, one-half size, one-fourth size, and so forth. See the examples in figure 1-1, views A and B. If the scale is shown as 1 = 2 , each line on the print is shown one-half its actual length. If a scale is shown as 3 = 1 , each line on the print is three times its actual length. The scale is chosen to fit the object being drawn and space available on a sheet of drawing paper. Never  measure  a  drawing;  use  dimensions.  The print may have been reduced in size from the original drawing. Or, you might not take the scale of the drawing  into  consideration.  Paper  stretches  and shrinks as the humidity changes. Read the dimensions on the drawing; they always remain the same. Graphical scales on maps and plot plans show the number  of  feet  or  miles  represented  by  an  inch. A fraction such as 1/500 means that one unit on the map is equal to 500 like units on the ground. A large scale map has a scale of 1 = 10´; a map with a scale of 1 = 1000´ is a small scale map. The following chapters of this manual have more information on the different types of scales used in technical drawings. Station Number A station on an aircraft may be described as a rib (fig.  1-3).  Aircraft  drawings  use  various  systems  of station markings. For example, the centerline of the