Nicolas-Jacques Conte developed some techniques to control the proportion between clay and graphite in the manufacture of pencils; the classification consists of a scale from 1 (hard) to 4 (soft); subsequently the English developed a scale that goes from B (black, i.e. soft) to H (hard, i.e. hard); the two scales will unify in the current scale which goes like this: (+hard) 9H … 2H, H, F, HB, B, 2B, … 9B (+soft)
