The different degrees of hardness and what you use them for
Different degrees of hardness are produced depending on the ratio of clay to graphite. More clay gives the lead greater strength and more graphite results in darker abrasion. The MONO 100 line ranges from 9H (hard/hard) in various grades to HB (hardblack/hard black), F (firm) and 6B (black/black). As there is no fixed definition for the different degrees of hardness, an HB pencil can differ from one manufacturer to another.
A different ratio between clay and graphite results in different degrees of hardness