Pencils are made with many grades of lead hardness. See Figure 4-1. Hard leads can produce thin, light lines and are well suited for the accuracy requirements of board-type drawings, but are usually too light for sketching. The soft leads produce broad, dark lines, but tend to smudge easily if handled too much.
The choice of lead hardness is a personal one. Some designers use 3H leads very successfully; others use HB leads with equally good results. You are probably used to a 2H lead, as this is the most commonly available. Start sketching with a 2H lead, and if the lines are too light, try a softer lead; if they are too dark, try a harder lead until you are satisfied with your work. Pencils with different grades of lead hardness are available at most stationery and art supply stores.
Most sketching is done in pencil because it can easily be erased and modified. If you want the very dark lines that ink produces, it is recommended that you first prepare the drawing in pencil and then use a pen to trace over the lines that you want to emphasize.