These are obvious… yes?
They come in softnesses (or is it hardnesses?)… from 9H or so through to 2H, H, F, HB (in the middle) then B, 2B… to 9B perhaps.
Normal pencils from the shop are made using the ‘Conte’ process. Rather than having pure pieces of graphite (as may be obtained from specialist are outlets) the ‘Conte’ processes involved grinding graphite and mixing it with ground clay. The resulting mixture is baked to make a usable pencil core. Varying the ratio of clay to graphite makes a pencil of differing hardnsses. It also allows manufacturing control so that a ‘product’ can be consistently hard or soft as a pencil. (http://www.pencilpages.com/articles/grades.htm)
Pencils are very friendly – easy to carry around; cheap to obtain; ubiquitous; dry.
I’ve drawn a train conductor with a hat 4 times – as a line drawing, a tonal drawing, a combination of line and tone (maing it look quite styalised) and a ‘compositional’ sketch – showing an arrangement of figures. These were all done fast – one of the benefits of pencils they allow you to express ideas very fast. Sketching with pencils then becomes a preliminary to many of forms of creation… not just art, but also in engineering and other art and design genres.
This sketch “Romance of Steam” I did just to look at steam engines as a contrast the to HS2. The image may not be functional in the sense that I might use it directly by in drawing it and considering the era of steam and the people of the time I considered things like the parallel situation then and now.