The ancient Greeks thought that the most pleasing proportions for a rectangle were those in which the rectangle's sides were in the ratio of about 1.618 to 1. This number is called the golden section or golden ratio and a rectangle with those proportions is called a golden rectangle.
Leonardo da Vinci referred to the golden ratio as the "divine proportion". There are several features of his most famous painting, the Mona Lisa, to which a golden rectangle can be fitted. See how many of these you can find by varying the orientation, position, and size of the rectangle.