HB is a scale for two different measurements for graphite.
The H is for the hardness, and the B is for the softness.
HB is your standard #2 pencil, falling within the middle of the scale.
From there, hardness is to the left and softness is to the right.
A 4B, for example, is softer than a B, but also darker in shade.
A 3H, on the other hand, is harder than a 2H, but also of a lighter shade.
A good way to draw any type of head in perspective is the Loomis method.
A step by step guide of the method: http://www.stanprokopenko.com/blog/2009/05/draw-head-any-angle/
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