Non-standard but well-accepted measure
of the hardness
of pencil leads (made of different proportions of graphite and clay), commonly ranging from 6B (maximum graphite, hence softest) to 6H (least amount
of graphite, hence the hardest) with HB (roughly equal amounts of graphite and clay, hence medium soft/hard) in the middle
. Pencil hardness is used in indicating the toughness
of surface coatings by testing
which number pencil-lead can scratch it.