Turn a box so that one corner points at you. Now no face is flat to you. The edges on the left side go back to the left, and the edges on the right side go back to the right. That takes two vanishing points, both on the horizon line: two-point perspective.
Key idea
In two-point perspective, a box seen corner-first has two vanishing points on the horizon line, one for each side. Vertical edges stay vertical.
Why it works
A box has three sets of parallel edges. In one-point perspective, two sets (across and up-down) are flat to you, so only one set needs a vanishing point. Turn the box and the across edges now go away from you too, to the left and to the right, so they need a second point. The up-down edges are still flat to you while you look straight ahead, so they stay vertical.
Worked example: A building on a corner
You stand on a street corner and look at a building's corner. How do you set up the drawing?
- Draw the horizon line at your eye level, about the height of a person's eyes on the building.
- Put one vanishing point far to the left and one far to the right on that line.
- Draw the nearest corner as one vertical line.
- From its top and bottom, draw lines to each vanishing point. Add the far vertical edges.
Answer: A corner line, two vanishing points far apart, vertical edges kept vertical.
Common mistake
Placing the two vanishing points close together makes boxes look stretched and sharp-cornered, as if seen through a fisheye lens. Put them far apart, often off the edges of the paper. Tape scrap paper beside your drawing so you have room.
In Caillebotte's Paris Street; Rainy Day, the building at the corner of the two streets shows two sides going back in different directions.