Phi - the golden ratio - 1.6180339887

The golden ratio, also known as the divine proportion, golden mean, or golden section, is a number often encountered when taking the ratios of distances in simple geometric figures such as the pentagon, pentagram, decagon and dodecahedron. It is denoted , or sometimes .

Related to this, is the Fibonacci sequence, which when arranged in a spiral it is said to have, in art, the perfect proportion, as it keeps the golden ratio.: http://en.wikipedia.org/wiki/Fibonacci_sequence

The golden angle is a variant of this ratio, most notably, the golden angle is the angle separating the florets on a sunflower.