Jim Gollan @ OCA Art of Photography: Lines (2.2b)

To show how:

Diagonals are often created from linear perspective (linear perspective adds depth to an image)
Diagonals add movement to an image

Creating diagonals in photography depends largely on camera angle and perspective: diagonals are less common in real life than vertical or horizontal lines
Diagonals create a stronger sense of movement and direction than vertical and horizontal lines do: (a) they contrast more strongly with the edge of the frame and (b) they are associated with being physically unstable, even of falling
Wide angle lenses used close to an edge or a surface creates strong diagonals (sense of perspective), whilst telephoto lenses used at an angle to parallel lines, edges, rows of objects, can give a powerful diagonal effect by compression and repetition

Examples

Straight edges (e.g. flat top of a building, top of a wall) become diagonal lines if you photograph along them towards the distance, due to perspective in 2D images
Vertical or horizontal lines in a close up shot become diagonal if you rotate your camera
Vertical or horizontal lines in long shots taken from a high point become diagonal if you move the camera sideways

Take 4 photographs with strong diagonals.

Used cars at Brooklands, Weybridge

Row of objects shot with a telephoto lens
with frame cropped to 2:1 ratio

Statue of Sir Joshua Reynolds outside Royal Academy, London

Focus from a low viewpoint
on arm of statue creates
a diagonal line

Late evening strollers along South Bank, London

Setting sun throws diagonal shadows on
the paving stones of the embankment

Promenade at Sidmouth, Devon

Telephoto lens used at an acute angle to
the terraced buildings and railings creates
a diagonal subject

Jim Gollan @ OCA Art of Photography