Diagonal lines in an image have a greater sense of movement and although they can be found occurring in man made structures or occurring naturally, they are also easy to create by changing the camera angle or camera lens.
This first image is straightforward and shows diagonal lines created by a man made object. In this case a pedestrian walkway.
In the image below, taken with a Sigma 10-20mm wide angle lens, the horizontal lines of the railings each side of the footpath recede into the distance. The linear perspective is enhanced by the low angle of view and the lens used. This has the effect of turning what normally would be horizontal lines into diagonals. The effect of the diagonals in this case is to enhance the feeling of depth in the image.
Similarly by changing the angle of view with a standard lens or simply cropping an image, diagonals can be created as in the example below.
There are other examples of diagonal lines within the course notes which are also incomplete. If I had the chance to take this image again I would have taken it from a lower viewpoint so that the lines continued further into the image.
The following are links to further images which show strong diagonals.
Kertesz,Andre Fork 1928. http://en.wikipedia.org/wiki/File:Kertesz_The_Fork.jpg
Abbott,Berenice Manhattan Bridge 1936.http://www.artnet.com/usernet/awc/awc_workdetail.asp?aid=425932858&gid=425932858&cid=195821&wid=426064541&page=6
Laughlin,Clarence John The Dynamics of Cylinders 1946. http://www.bridgemaneducation.com/ImageView.aspx?result=0&balid=374403