Rule of Thirds / Golden Ratio

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Golden Ratio

The Golden Ratio, or phi, is perhaps the most perfect number in all mathematics. It is equal to the squareroot of 5 plus 1, divided 2. 
(Sqrt(5)+1)/2 = 1.618033988749895 

Many artists and architects have proportioned their works to approximate the golden ratio—especially in the form of the golden rectangle, in which the ratio of the longer side to the shorter is the golden ratio—believing this proportion to be aesthetically pleasing (see Applications and observations below). Mathematicians since Euclid have studied the properties of the golden ratio, including its appearance in the dimensions of a regular pentagon and in a golden rectangle, which can be cut into a square and a smaller rectangle with the same aspect ratio.