There they are, the eyes of a rectangle. Many compositions begin by seizing one of these points and sticking something real important there. You may have played tic-tac-toe and knew which square to grab if you went first. Its a little like that. However its so easy to do that you can do this design too often, a roomful of power spot designs can look pretty monotonous. It also tends to give a formal 19th century look. That may be just what you want, or maybe it is not.
As I said the other night I felt like I was just mindlessly starting every composition this way a few years back and now I am on the lookout for that fault. I still use it, but I try to design by pattern instead,That is arranging interlocking shapes of varying size and value across my canvas. I think I get more varied designs that way and they look less predictable.
My friend Bob, who knows his math, wrote in the comments the following;
On a canvas with the common 3:4 ratio (12x16, 18x24, etc.), the diagonal is 5 units. With a little trigonometry, you can figure out that the power spot is 1.8 units from the closer corner along the diagonal. In other words, the spots are at 0.36 of the diagonal length from each corner. If you drop a perpendicular from a power spot to the long dimension of the canvas (the 4 unit dimension), it is 1.44 units from the edge, which is 0.36 of the long dimension. Each spot is 1.08 units from the top or bottom, which is 0.36 of the short dimension (the 3 unit dimension). The recurrence of the fraction 0.36 in all these dimensions is fascinating -- I see the makings of a cult. The fraction 0.36 is roughly one third (0.33), so the spots lie very close to the intersections of lines that divide the canvas into thirds.
I had to read that several times (once lying down) in order to understand it.
Above (courtesy of artrenewal.org) is a John Constable, The White Horse. The major tree is placed on the eye in the upper left of the rectangle. That gives it a place of great prominence within the design. There is also a white cow marking the eye on the lower right.
Here is the wonderful Bouguereau from the Clark Art Institute in Williamstown, Massachusetts. If you have never been to that museum, it is well worth a trip to western Massachusetts to see it. There are many masterpieces there, including this, Bouguereau's finest painting. He has placed a figure of a nymph at both the upper left and lower right eye. The stern of the nymph on the right is moored securely right on that point.That may be what gives this painting such a broad based appeal. Incidentally, notice how nicely the raised arm of the woman on the upper right leads us up and to an easy exit after we have viewed the rest of the scene. It also repeats rhythmically the leg of the figure on the left,and also the leg on the lower right, giving a sort of pinwheel design to the painting.