Tomorrow, March 14, is 3/14 [in month/year form] and so we celebrate Pi Day! And since I’m teaching History of Math for the first time in rather a lot of years, I’m thinking that the perfect topic is the history of the symbol.

But first, what isĀ š¯¯…?Ā The idea is that no matter how big or small a circle, the circumferenceĀ is always a little more than 3 times as large as the diameter:Ā that ratio is about 3.14 in decimal terms.Ā Because it’s a ratio, the first symbols were also written as ratios:Ā William Oughtred called itĀ š¯¯…**/Ī´** in 1647, and while he didn’t explain what either of the terms meant, sinceĀ š¯¯…Ā is the Greek p it likely stood forĀ *peripheryĀ *(according to my source, though I think of perimeter myself when I see it); likewise,Ā **Ī´**Ā is the Greek d and likely stood forĀ *diameter*.Ā Several otherĀ mathematicians adopted this notation.

The first person to use a single symbol to represent this ratio was Johann Christoph Sturm, who in 1689 referred to it with the letter *e*. Wait, what? (Double check.) Well that is something I didn’t know before. Cool! But using *e* didn’t catch on, and less than twenty years later, in 1706, William Jones used the symbol š¯¯… for this same ratio. No explanation as to why, and also no consistency – he used the same symbol to mean other things earlier in the same book. This use of š¯¯… also didn’t catch on: other mathematicians continued to use other symbols for the ratio of a circumference to the diameter, and š¯¯… itself continued to be used for different mathematical numbers. But eventually, over the 1700s, its use caught on and so we have the well known symbol today.

Source: *A History of Mathematical Notations* by Florian Cajori

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