To add to your set of examples of horizontal and vertical lines, now take four photographs which use diagonals strongly.

 – AoP Coursebook 

I’d question the notion that diagonals are rare in man-made structures at any rate, as anywhere there is a vertical – if it needs some sort of rigidity – there is generally a diagonal support. However here my main thought is how easy it is for a diagonal to become unstable: I’ve tried # 3 (shot at Oxford Circus on the Victoria Line platform) all 4 ways up and find none of them quite have a straightforward sense of movement without any form of twist to it. Obviously, I need to pay more attention to how the sensor in my camera is aligned with the diagonal lines, if this is not to happen again…