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It all depends. I think I'd feel more comfortable if the horizontal lines on your example were angled slightly towards a distant VP.

Let's not labour the point. We'll agree to disagree!

I agree with Egg, I'm afraid. My minimalist example did not show a side face. If it had I should have shown another vanishing point off to the left.

Yes, indeed in my example and Wizard's example, there should be a second vanishing point off to the left and no parallel lines on the side face.

A big 'Doh on my part. Thanks, Egg.

I think what is required is "Horizontal edges on the facing plane should be directed towards the vanishing point. Horizontal edges on side plane remain horizontal."

Vertical lines are indeed...

Maybe this effect is what is required?