microform  reproduction.  There  are  two  types:  fired and extruded. They are similar in material content to plastic fired lead, but they are produced differently. The main drawback with this type of lead is that it does not hold a point well. PENS Two types of pens are used to produce ink lines: the   ruling   pen   with   adjustable   blade   and   the needle-in-tube type of pen (fig. 2-2). We include the ruling pen here only for information; it has been almost totally replaced by the needle-in-tube type. The second type and the one in common use today is a technical fountain pen, or needle-in-tube type of pen. It is suitable for drawing both lines and letters. Figure  2-5—Adjustable  triangle. The draftsman uses different interchangeable needle points to produce different line widths. Several types of these pens now offer compass attachments that allow them to be clamped to, or inserted on, a standard compass  leg. DRAWING AIDS Some  of  the  most  common  drawing  aids  are protractors, triangles, and French curves. A protractor (fig. 2-3), is used to measure or lay out angles other than  those  laid  out  with  common  triangles.  The common triangles shown in figure 2-4 may be used to measure or lay out the angles they represent, or they may  be  used  in  combination  to  form  angles  in multiples of 15°. However, you may lay out any angle with an adjustable triangle (fig. 2-5), which replaces the protractor and common triangles. The French curve (fig. 2-6) is usually used to draw irregular curves with unlike circular areas where the curvature is not constant. TYPES OF LINES The lines used for engineering drawings must be clear  and  dense  to  ensure  good  reproduction.  When making additions or revisions to existing drawings, be sure  the  line  widths  and  density  match  the  original work. Figure 2-7 shows the common types of straight Figure  2-6.—French  (irregular)  curves. 2-3