# Why is Pi Pi? **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2014, 7:57pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806 "2014-01-14T19:57:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [January 14, 2014, 7:57pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/1 "2014-01-14T19:57:15Z")
In other words, why was the letter Pi chosen to symbolize the ratio of a circle’s circumference to its diameter? As opposed to say Omicron or Rho? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 14, 2014, 7:59pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/2 "2014-01-14T19:59:01Z")
It stands for “perimeter”. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [January 14, 2014, 8:03pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/3 "2014-01-14T20:03:49Z")
But then why is perimeter (circumference) a multiple of 2pi? ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [January 14, 2014, 8:05pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/4 "2014-01-14T20:05:00Z")
Yeah, probably, but nitpick: _periphery_. [Adoption of the Symbol PI](http://en.wikipedia.org/wiki/Pi#Adoption_of_the_symbol_.CF.80) ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [January 14, 2014, 8:06pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/5 "2014-01-14T20:06:01Z")
> [@The Great Sun Jester](#): > > But then why is perimeter (circumference) a multiple of 2pi? compared to the radius. It’s only 1pi as big as the diameter. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [January 14, 2014, 8:07pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/6 "2014-01-14T20:07:36Z")
O\_o I knew that. Fuggin pi…how does it work? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 14, 2014, 8:16pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/7 "2014-01-14T20:16:47Z")
It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 14, 2014, 8:24pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/8 "2014-01-14T20:24:54Z")
> [@Little\_Nemo](#): > > In other words, why was the letter Pi chosen to symbolize the ratio of a circle’s circumference to its diameter? As opposed to say Omicron or Rho? In George O. Smith’s _Lost Art_, a story from his _Venus Equilateral_ series, he says that the lost ancient Martian civilization represented Pi as a circle with a double dot. A single dot would make more sense, to me. To answer **great Sun Jester** , all linear measures of a figure scale at the same rate, so what’s really important are the ratios, since those will be the same at any scale. Pi is the ratio between the circumference and the diameter, and shows up in surprising and (at first) unexpected mathematical relations, such as the results of infinite sums, the results of Cauchy integrals, relations with imaginary numbers, as well as in engineering problems like relations of the bending of simply supported beams, resistances measured between points on an infinite network grid of resistors, and similar such stuff. I’m sure people here will tell you more than you wish to know about any of these. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [January 14, 2014, 8:37pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/9 "2014-01-14T20:37:52Z")
> [@CalMeacham](#): > > I’m sure people here will tell you more than you wish to know about any of these. Or we can just send them to [this portion of the relevant Wikipedia article](https://en.wikipedia.org/wiki/Pi#Outside_mathematics). ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [January 14, 2014, 8:38pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/10 "2014-01-14T20:38:27Z")
> [@Chronos](#): > > It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. Here we go. 😃 > **[Pi has enemies.](https://boards.straightdope.com/sdmb/showthread.php?t=614230)** > > The mathematical constant pi is under threat from a group of detractors who will be marking “Tau Day” on Tuesday. Tau Day revellers suggest a constant called tau should take its place: twice as large as pi, or about 6.28 - hence the 28 June... > **[Mathematics: Oughtn't pi be twice what it is?](https://boards.straightdope.com/sdmb/showthread.php?t=582311)** > > It seems to me that when mathematicians consider circles, they normally think of them in terms of the radius. A “unit circle” is a circle with a radius of 1, not a diameter of 1. Angles are measured in terms of the radius, not the diameter. And yet,... ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [January 14, 2014, 8:45pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/11 "2014-01-14T20:45:38Z")
> [@Chronos](#): > > It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. I found a lot of things in my college trig course much easier once I started beginning all of my homework with _Let τ = 2π_. ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [January 14, 2014, 8:53pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/12 "2014-01-14T20:53:22Z")
> [@Chronos](#): > > It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. Qute video [http://http://www.youtube.com/watch?v=jG7vhMMXagQ](http://http://www.youtube.com/watch?v=jG7vhMMXagQ) ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [January 14, 2014, 10:25pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/13 "2014-01-14T22:25:44Z")
Okay, I guess I should have done more research. I knew the Greek word for circle (κύκλος) started with a K so I wrongly assumed that whatever the Greek word for circumference was, it would have the same root and also start with a K. But the Greek word for circumference is περιφέρεια, which starts with a π. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [January 14, 2014, 11:12pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/14 "2014-01-14T23:12:06Z")
> [@Chronos](#): > > It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. I think tau and pi are backwards. The larger number should have 2 stems. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [January 14, 2014, 11:16pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/15 "2014-01-14T23:16:38Z")
> [@Chronos](#): > > It has been pointed out that radius is a much more natural measure of a circle than diameter, anyway, and so the number that we ought to care about is 6.28…, now sometimes known as tau. Seems a superstitious concept and a cause of needless confusion to me. Tau will be a fleeting fad, forget it. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [January 14, 2014, 11:45pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/16 "2014-01-14T23:45:13Z")
> [@Isilder](#): > > Seems a superstitious concept and a cause of needless confusion to me. > Tau will be a fleeting fad, forget it. Superstitious? How, exactly? If anything is needlessly confusing, it’s using 2pi to represent 1 turn of the unit circle. Yeah, let’s measure angles in terms of the radius but base the constant on the diameter. That’s a good idea. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [January 14, 2014, 11:55pm UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/17 "2014-01-14T23:55:41Z")
> [@Little\_Nemo](#): > > Okay, I guess I should have done more research. I knew the Greek word for circle (κύκλος) started with a K so I wrongly assumed that whatever the Greek word for circumference was, it would have the same root and also start with a K. But the Greek word for circumference is περιφέρεια, which starts with a π. Since Pi is a Greek letter, and since much of geometry traces back to the ancient Greeks (Euclid, Archimedes, etc.), who certainly knew about the number, it’s natural to assume that pi is what the Greeks themselves called it. It’s surprising (or at least it was to me when I first learned it) that the use of the symbol is relatively recent compared to the knowledge of the number itself. As **bup** ’s link notes, it was popularized by Euler, as were many other bits of mathematical notation that we now take for granted (such as _e, i, f(x), Sigma_). ---
**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\ **Post date:** [January 15, 2014, 12:13am UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/18 "2014-01-15T00:13:35Z")
> [@standingwave](#): > > Superstitious? How, exactly? > > If anything is needlessly confusing, it’s using 2pi to represent 1 turn of the unit circle. Yeah, let’s measure angles in terms of the radius but base the constant on the diameter. That’s a good idea. [Nerdfight!!](http://www.xkcd.com/1292/) ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [January 15, 2014, 12:59am UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/19 "2014-01-15T00:59:59Z")
> [@j\_sum1](#): > > [Nerdfight!!](http://www.xkcd.com/1292/) [I don’t know what 1.75pi should be then](http://www.youtube.com/watch?v=PSh6SQd8UrI) ---
**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\ **Post date:** [January 15, 2014, 1:30am UTC](https://boards.straightdope.com/t/why-is-pi-pi/678806/20 "2014-01-15T01:30:01Z")
[chuckles] [Next page](https://boards.straightdope.com/t/why-is-pi-pi/678806.md?page=2)