# Why Is the Greek Letter *Pi* the Symbol for That Mathematical Ratio Anyway?

If the spike in lookups of *pi* at Merriam-Webster.com are any indication, many caught wind that today was Pi Day but weren't quite sure why. Arguably the nerdiest cause for celebration, Pi Day falls on March 14, or 3/14, because the mathematical value of π begins "3.14."

*Pi *is the English spelling of the sixteenth letter of the Greek alphabet. It means both "the symbol π denoting the ratio of the circumference of a circle to its diameter" and "the ratio itself: a transcendental number having a value rounded to eight decimal places of 3.14159265." This means that the outside measurement of a circle is a little more than three times its width, a ratio that is one of the most important constants in mathematics.

But why was *pi* chosen as the symbol for this ratio in the first place? Well, it was introduced by 18th-century Welsh mathematician William Jones, most likely, it's believed, because *pi* is the first letter of the Greek word for periphery (*periphérion*). Here's what it looks like in Greek: *περιϕέρεια*.

Nothing like a holiday that celebrates circumference!