Pencil makers manufacture Nos. 1, 2, 2.5, 3, and 4 pencils—and sometimes other intermediate numbers. The higher the number, the harder the core and lighter the markings.

The pencil “lead” is actually a mixture of graphite and clay. More clay in the mixture makes the lead harder. A harder lead has a higher number. Pencils are numbered from 1-3 but most people use a ...