Diagonal Lines
Lines running at an angle to the frame edges, which introduce energy and a sense of movement.
In depth
Horizontals suggest calm and verticals suggest strength because both align with the frame; diagonals suggest instability precisely because they do not. They are also the strongest cue for depth, since parallel lines receding from the camera converge diagonally in the image.
Examples in practice
Tilting the camera so a staircase runs corner to corner turns a static architectural detail into a dynamic one.