Beauty fads come and go, and eyebrow shaping is no exception. But what if there were a mathematical rule we could cite as hands-down the perfect shape? That’s the idea behind the patent recently awarded to inventor Anastasia Soare of Beverly Hills, California, for a “brow mapping technique” that hews closely to the so-called “golden ratio.”

The golden ratio is sometimes called the “divine proportion,” or in mathematical terminology, simply phi. Its value is usually given as 1.618, although in reality, it’s an irrational number with infinite decimal places, like pi. Phi describes an intriguing geometrical anomaly. If a line is divided into two unequal lengths, such that the ratio of the longer segment to the shorter segment is the same as the ratio of the whole line to the longer segment, then the resulting number will be something close to 1.618.

Phi was first mentioned in Euclid’s Elements around 300 BC. Geometric shapes are said to be in “divine proportion” if the ratios of their various sides closely resemble phi. The most common are the golden rectangle, the golden triangle, and the pretty spiral observed in a chambered nautilus shell.