The Leadenhall Building at 122 Leadenhall Street, wedge-shaped and tapering back to protect the sightline to St Paul's, which is exactly why it makes such a clean diagonal in a frame. It faces the Lloyd's Building across a small plaza, so you get old and new, glass and steel, in the same look.

Best time: clear afternoons for reflections and sky in the glass. Blue hour for the lit interior against a deep sky.

Look for: the tapering edge cutting across the frame as a diagonal, clouds and neighbouring towers reflected in the facade, and commuters crossing the plaza as small figures beneath it. Point up for pure geometry.