number of the next larger assembly of which the smaller unit or assembly is a part. The USED ON column  shows  the  model  number  or  equivalent designation of the assembled units part. FINISH MARKS Finish marks (P) used on machine drawings show surfaces  to  be  finished  by  machining  (fig.  1-6). Machining provides a better surface appearance and a better fit with closely mated parts. Machined finishes are NOT the same as finishes of paint, enamel, grease, chromium  plating,  and  similar  coatings. NOTES AND SPECIFICATIONS Blueprints  show  all  of  the  information  about  an object  or  part  graphically.  However,  supervisors, contractors,  manufacturers,  and  craftsmen  need  more information that is not adaptable to the graphic form of presentation. Such information is shown on the drawings as notes or as a set of specifications attached to the drawings. NOTES are placed on drawings to give additional information to clarify the object on the blueprint (fig. 1-2). Leader lines show the precise part notated. A SPECIFICATION is a statement or document containing a description such as the terms of a contract or details of an object or objects not shown on a blue print or drawing (fig. 1-2). Specifications describe items  so  they  can  be  manufactured,  assembled,  and maintained  according  to  their  performance  require- ments. They furnish enough information to show that the item conforms to the description and that it can be made without the need for research, development, design engineering, or other help from the preparing organization. Federal  specifications  cover  the  characteristics  of material  and  supplies  used  jointly  by  the  Navy  and other  government  departments. LEGENDS AND SYMBOLS A legend, if used, is placed in the upper right corner of a blueprint below the revision block. The legend explains or defines a symbol or special mark placed on the blueprint. Figure 1-2 shows a legend for an electrical plan. THE MEANING OF LINES To read blueprints, you must understand the use of lines. The alphabet of lines is the common language of the technician and the engineer. In drawing an object, a draftsman arranges the different views in a certain way, and then uses different types of lines to convey information. Figure 1-6 shows the use of stan- dard  lines  in  a  simple  drawing.  Line  characteristics Figure 1-6.—Use of standard lines. 1-7