Writing hardness is determined by the mix ratio of graphite and clay: The greater the graphite content the softer the lead and the higher the proportion of clay the harder the lead.  The choice of hardness required by the user of the pencil is determined by the type of writing or drawing work, by the paper surface and by the “heaviness” of hand of the user. (A heavy hand might prefer softer leads, and a light hand may work well with relatively hard lead qualities.) 

The number and lettering printed on the pencil represents the degree of lead hardness.  This indicates how soft or hard the lead is and how dark or light the mark is on the paper. Again this all has to do with the mixing ratio of clay and graphite.

Faber-Castell applies the following designations:

H  = hard

B  = black = soft

F  = firm

HB = hard black = medium hard

Degrees of lead hardness alternatively expressed in numbers:

1          2          2 ½       3          4

2B        B          HB       H         2H

Degree B is Equivalent to a No.2