Crossing Lines/diagonals are actually again
another simplification of the Golden Mean. The basic idea is to provide a sort of "guideline" for the eyes to follow.
It is a good idea to place the start or end of such a line to one of the
extreme edges. The classical approach states that the upper left edge is
the best starting point because most humans start to traverse a picture
from here on. However, it cannot hurt to break this rule (see 2nd picture).
Just a straight line would be pretty boring though so there should be some
sort of disturbance in the picture.
The following picture shows a focus point
where many lines find together so there are enough of directions for the
eyes to follow making the picture interesting.
by Horst Scheider
The next picture has two anchors - the
boat and the sun shade. These provide just the right amount of disturbance
for this otherwise very symmetrical composition.
by Horst Schneider