Simple Three Point Perspective
By Helen South, About.com Guide
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Constructing the Box
Next lightly draw some construction lines. Begin at the left vanishing point, straight to about 1/3 of the way up the vertical line, back down to the right vanishing point. Then another, from the left vanishing point, to about 3/2 of the way up, and then straight to the right vanishing point. Those mark the top and bottom egdges of your box. Now draw two lines from the top vanishing point - these can be as wide or narrow as you like, but something like the ones in the example; these will mark the front left and back right edges of the box.