Pi meaning
Pi is how many diameters fit around a circle. The ratio is ancient. The letter π is William Jones’s, in 1706. Archimedes did not say pi equals 22/7. He proved it sits between two fractions.
How many diameters fit around a circle. The letter came later, from a printer’s book.
The name. William Jones used π in 1706, for the Greek perimetros, the periphery. Euler’s habit made the letter universal.
Where it comes from
Egypt and Babylon already used decent approximations: the Rhind papyrus works out near 3.16, a Babylonian tablet uses 25/8. Archimedes did not declare the ratio equal to 22/7. He trapped it between 223/71 and 22/7, which is a proof, not a slogan. The symbol is William Jones, Synopsis Palmariorum Matheseos (1706), then Euler. The digits do not encode a message. They do not terminate, and they do not repeat.
Geometry
A ratio that will not finish
Lambert showed it is irrational (1768). Lindemann showed it is transcendental (1882), which is why the circle cannot be squared with straightedge and compass: you cannot build a square of equal area. The old puzzle died of a theorem, not of discouragement. 22/7 remains a useful bound. It is not the number.
Modern
A letter that became a mascot
March 14 is pi day because Americans write the month first. That is a joke about a calendar, and a good one, and it is not a feast inherited from a cult of the circle. The enso is a circle with a different job. This sign is a ratio, and then a Greek letter chosen because it was free.
Near this
Sources
- Archimedes, Measurement of a Circle
- William Jones, Synopsis Palmariorum Matheseos (1706)
- Lambert (1768), irrational; Lindemann (1882), transcendental
Named when the entry depends on a text, a date, or a standard. Catalog numbers are not invented.
Your margin
Private. It stays in this browser and never joins the canon.
symbol.wiki, edited as Sigilla. Last reviewed 2 October 2026. A reading of public tradition, not a ruling.