Where are the right angles on the arc de triomphe?

Answer 1

Each of the four pillars has a rectangular shape at ground level. There you could find a square angle (vertical side of the pillar + horizontal surface of the ground), and a square angle at each turn of every pillar. At the top of the arc you'll find another horizontal surface which is at a square angle with the vertical sides, plus four square angles at the turn of every side.