This exercise is about two-point or angular perspective. Basically an object or building that does not have a straight edge facing on to a viewer has an angular perspective – there can be two vanishing points.
To explore this concept I made a line drawing in pen of a building seen corner on. I established my eye level first and checked the horizontal and vertical lines. Once I had finished my drawing I used a ruler to draw onto my drawing the receding lines. I found the exercise a challenge but felt I still need to practice drawing in this way.