H and HB

Medium line density.

This is the most popular hardness for lead.

It has a good strenght and smooth feeling.

Aslo, it is ideal for accounting and general writing.

2H

Light to very light line with exceptional tensile strength.

This one is good for drafting and engineering.

* Tensile strength is the resistance of a material to a force tending to tear it apart and it is measured as the maximum tension that the material can withstand.

B and 2B

Dark to very dark line density. 

Therefore, it is very easy to read and will transmit through fax machine, copier and computer scanner.

This also can be used for shading and sketching.