The Greek letter φ, phi, is used in mathematics to represent the Golden Ratio. Also known as the Golden Mean, Golden Section, Divine Proportion, etc., its value is evident in the geometries of pine cones, flowers, seashells, the spirals of galaxies, and countless other examples. The principle of this ratio has been deliberately used in art, architecture, and even music. This “Divine Proportion” is what Leonardo Da Vinci was exploring in his sketch of the “Vitruvian Man” (and all of the illegal body exhuming he did to investigate it.)

φ is an irrational number, meaning that it cannot be represented as a fraction of whole numbers. In its decimal form, 1.6180339887…, the number sequence never ends and never repeats. This means that it is infinitely precise. The idea of such irrational numbers may be most familiar to people in the number π, pi, 3.1415926535…, the ratio of any circle’s circumference to its diameter. π, however, is both irrational and transcendental as a number. That is, though φ can be algebraically represented as (1±√5)/2 [1 plus or minus the square root of 5 divided by 2], there is no algebraic representations of π.

Famously, φ is related to the Fibonacci Sequence—0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233…—in that by taking any number and dividing it by the value that preceded it in the sequence, the value of the Golden Ratio is approximated. The further down the line of Fibonacci numbers this operation is carried out, the more accurately the quotient approaches φ.