“Fashion is architecture: it is a matter of proportions.” – Coco Chanel

They say that beauty is in the eye of the beholder — but a good mathematician would tell you that beauty lies in a ratio known as the Divine Proportion. Phi (ɸ), the 21st letter of the Greek alphabet, helps explain this proportion. While phi is widely accepted as a fraternity or sorority symbol, in math it represents an irrational constant number. (To those who haven’t taken math in a while, an irrational number is one with an infinite number of digits after the decimal place. You might recall the number Pi (π) is one such number.)

Phi helps explain the ratio obtained when an area is split into two segments. The first is a square (with equal height and width); the second is a rectangle (with the same width). See illustration below.

This illustration shows the equivalence ɸ/1 = 1/ ɸ-1, so ɸ is approximately 1.618. (Cross multiply, to get the quadratic equation ɸ2 – ɸ – 1 = 0. Solve from there.) This is the relationship known as the Divine Proportion, but has also been given many other names including the Golden Ratio and the Golden Mean.

The area, pronounced “fee,” is considered the building block of the Great Pyramids in Egypt, as well as the Parthenon in Greece. History proves architects have deliberately used phi in their designs. It has not yet been proven why this ratio is desired, but it has been proven that it’s aesthetically pleasing. tumblr_luyyb1W1vv1r0kkgko1_500

If fashion is indeed architecture, as Chanel suggests, why don’t couturiers use the same proportions — phi to be specific — that architects use in their designs?

Some analysts believe Coco Chanel did use phi in her designs. She claimed her revolutionary skirt length was meant to cover knees, a part of the human anatomy she openly disliked, when in reality the proportions of her clothes were mathematically planned around phi.

“All of the great fashion couturiers used the Divine Proportion, perhaps by instinct,” Antonio Gonzalez de Cosio, fashion editor and author, told USA TODAY College in an email. Take for example, “Balenciaga with his architectonical designs [or] Dior creating his new look with very small waist lines in proportion to the body.” Couture is about clothes with “harmony of form and of balance.”

Read more at USA Today College.