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Drawing the Mouth - Structure

H South from photo courtesy O. Chignolli, licensed to About.com, Inc.
A common problem in drawing portraiture is misalignment of the nose, mouth and eyes. Avoid this by roughing in your structure first - notice that a line through the eyes, a line through the base of the nose, the line through the mouth, and a line through the chin, all project back to a vanishing point - front-on, these lines would be parallel; at an angle, they are (slightly) in perspective.
More Portrait Drawing Lessons
More on Drawing the Head and Face