Think of rectangle with a length roughly one and a half times its width--this ratio of 1.6:1 is called the Golden Ratio. Often referred to as "nature's number", this ratio shows up in everything from plants and animals to people and planets.
The Egyptians used the golden ratio in constructing the Pyramids. The Ancient Greeks used it in much of their architecture, including the Parthenon. During the Renaissance artists used the golden ratio extensively in the composition of their paintings, as it wasconsidered the most beautiful proportion. Leonardo da Vinci called it "The Divine Proportion".