# Pi (letter)
**Pi** (πι; uppercase Π, lowercase π) is the sixteenth letter of the [Greek alphabet](https://www.edgepedia.org/greek-alphabet), representing the voiceless plosive sound /p/ and pronounced /paɪ/ in English[1](https://www.oxfordlearnersdictionaries.com/us/definition/english/pi). It is best known outside Greek through its lowercase form, which since the early eighteenth century has served as the symbol for the ratio of a circle's circumference to its diameter, about 3.14159[1](https://www.oxfordlearnersdictionaries.com/us/definition/english/pi). As a consonant letter it transliterates into the [Latin alphabet](https://www.edgepedia.org/latin-alphabet) as p[2](https://www.collinsdictionary.com/dictionary/english/pi).
| Key fact | Detail |
|---|---|
| Position | 16th letter of the Greek alphabet, written Π (uppercase) and π (lowercase)[1](https://www.oxfordlearnersdictionaries.com/us/definition/english/pi) |
| Sound and pronunciation | Voiceless bilabial plosive /p/ in Greek; /paɪ/ as an English word[2](https://www.collinsdictionary.com/dictionary/english/pi) • [1](https://www.oxfordlearnersdictionaries.com/us/definition/english/pi) |
| Origin | From the Phoenician letter pe, meaning mouth[3](https://www.divinityatlas.com/alphabet-letters/pi) |
| Numeral value | As a numeral in the Greek alphabetic system, pi stands for eighty[4](https://www.divinityatlas.com/alphabet-letters/pi-2) |
| Descendant | The Latin letter P descends from Greek pi through the Etruscan adaptation of a Western Greek alphabet[3](https://www.divinityatlas.com/alphabet-letters/pi) |
| Mathematical symbol | π for the circle constant first used by William Jones in 1706 and popularized by Leonhard Euler[5](https://time.com/4699479/pi-day-history-origins/) |
## Form and sound
In Greek, pi represents the /p/ sound, as in the initial consonant of English *path*[6](https://www.greece.com/info/language/greek_alphabet/Pi/). Because it looks like the Latin letter P, it is a recurring source of confusion for readers who approach Greek through the [English alphabet](https://www.edgepedia.org/english-alphabet), since the Greek letter rho (Ρ), not pi, carries the r sound[7](https://letterlineage.com/letters/p-r/).
The familiar printed form Π does not stand for every ancient version of the letter. Archaic pi had several forms; one early shape has a long left leg and a short right one[7](https://letterlineage.com/letters/p-r/) • [8](https://www.mannafest.faith/greek/pi). Early Greek scribes varied considerably and wrote in either direction, left to right or right to left, before left-to-right writing became the norm[8](https://www.mannafest.faith/greek/pi).
## Origins and descendants
The Greeks borrowed and modified their alphabet from the Phoenicians, and pi descends from the Phoenician letter pe, whose name means mouth[3](https://www.divinityatlas.com/alphabet-letters/pi) • [8](https://www.mannafest.faith/greek/pi). The Semitic form of the sign probably carried no meaning of its own[6](https://www.greece.com/info/language/greek_alphabet/Pi/).
**Lineage into Latin.** The Latin letter P descends from Greek pi, reaching Latin through the Etruscan adaptation of a Western Greek alphabet[3](https://www.divinityatlas.com/alphabet-letters/pi). In the Etruscan alphabetic sequence, pi corresponds to the letter for the sound p, a separate sign from the one that became Latin R[7](https://letterlineage.com/letters/p-r/).
As a numeral, pi has the value eighty in the Greek alphabetic numbering tradition, closing the tens before rho opens the hundreds[4](https://www.divinityatlas.com/alphabet-letters/pi-2).
## Mathematical and scientific uses
The lowercase π became the standard symbol for [Archimedes' constant](https://www.edgepedia.org/pi), the ratio of a circle's circumference to its diameter. [William Jones](https://www.edgepedia.org/william-jones), a Welsh mathematician, first used the Greek letter for this number in 1706, and [Leonhard Euler](https://www.edgepedia.org/leonhard-euler) popularized the notation a few decades later[5](https://time.com/4699479/pi-day-history-origins/). Jones's choice is thought to echo the Greek word *perimetros* (perimeter), since pi is the first Greek letter in the words "periphery" and "perimeter"[3](https://www.divinityatlas.com/alphabet-letters/pi) • [9](https://apnews.com/article/ca302b43d7f64acaf6a234f4206206a1).
Both cases of the letter carry technical meanings. The uppercase Π represents the product operator in mathematics and a plane[6](https://www.greece.com/info/language/greek_alphabet/Pi/). The lowercase π represents, among other things, the prime counting function, profit in microeconomics and game theory, inflation in macroeconomics, the state distribution of a [Markov chain](https://www.edgepedia.org/markov-chain), the pi bond of chemistry, and the pion (pi meson) in particle physics[6](https://www.greece.com/info/language/greek_alphabet/Pi/).
The circle constant itself is irrational: its digits continue infinitely without repetition or pattern, and computations with computers have produced trillions of digits with no end[9](https://apnews.com/article/ca302b43d7f64acaf6a234f4206206a1). Archimedes of Syracuse is credited with the first calculation of pi to the familiar value of about 3.14[10](https://time.com/4250116/pi-day-2016/).
## Cultural afterlife
Because 3.14 matches the calendar date March 14 written in the United States as 3/14, that day is celebrated as [Pi Day](https://www.edgepedia.org/pi-day). The [United States House of Representatives](https://www.edgepedia.org/united-states-house-of-representatives) passed a ceremonial resolution designating March 14 as National Pi Day in 2009, encouraging schools to observe the day with activities that teach students about pi and mathematics[11](https://time.com/4697605/pi-day-birthday/).
## References
1. [pi noun, Oxford Advanced Learner's Dictionary](https://www.oxfordlearnersdictionaries.com/us/definition/english/pi)
2. [Pi definition and meaning, Collins English Dictionary](https://www.collinsdictionary.com/dictionary/english/pi)
3. [Pi π - Alphabet Letters, Divinity Atlas](https://www.divinityatlas.com/alphabet-letters/pi)
4. [Pi ⲡ - Alphabet Letters, Divinity Atlas](https://www.divinityatlas.com/alphabet-letters/pi-2)
5. [This Is How the Number 3.14 Got the Name 'Pi', TIME](https://time.com/4699479/pi-day-history-origins/)
6. [Greek Letter Pi (Πι - Π), Greece.com](https://www.greece.com/info/language/greek_alphabet/Pi/)
7. [P, R & Greek Rho - Letter Lineage](https://letterlineage.com/letters/p-r/)
8. [Pi (Ππ): the 16th Greek letter, MannaFest](https://www.mannafest.faith/greek/pi)
9. [What's Pi Day all about? Math, science, pies and more, AP News](https://apnews.com/article/ca302b43d7f64acaf6a234f4206206a1)
10. [Why Pi Day 2016 Is Special, TIME](https://time.com/4250116/pi-day-2016/)
11. [Find Your Birthday Hidden in Pi, TIME](https://time.com/4697605/pi-day-birthday/)
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*Topic: Encyclopedia › Society and history › Languages and linguistics › Writing and notation systems › Mediterranean, Italic, and undeciphered ancient scripts*
*Initially written Sep 17, 2026 · Reviewed: Oct 25, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 25, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license