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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.

1. The eyes are halfway down the head.
2. The space between the eyes is approximately the width of an eye.
3. The head is about five eyes wide.
4. The nose starts from the eyes and ends halfway down between the eyes and the bottom of the face.
5. The edges of the nostrils line up with the tear ducts of the eyes.
6. The mouth is just a bit above halfway between the mouth and the bottom of the face.
7. The corners of the mouth line up with the pupils of the eyes.
8. The ears start from the centerline of the eyes and go down until the bottom of the nose.

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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