Around the world, there are several roughly polygonal coins. Here's an example:

English Coins

One thing you'll notice is that they all have an odd number of sides. It turns out that this is universally true for modern polygonal coins (if you have an extant example where this is not true, please post it!!)

And it turns out that there's a perfectly sensible reason for this.

Why do modern polygonal coins have an odd number of sides?

Edit: I've been convinced by the various answers and comments that my statement is not correct and many coins around the world are, in fact, genuine polygons. My inspiration for this puzzle (if it's really a puzzle) comes from here.

  • 3
    $\begingroup$ Have a look here and here $\endgroup$
    – ielyamani
    Commented Apr 1, 2019 at 22:15
  • 4
    $\begingroup$ Not an answer, but almost a contradiction: Pieces of eight $\endgroup$
    – humn
    Commented Apr 2, 2019 at 1:57
  • 3
    $\begingroup$ Not universally true. Australian 50c piece is ten-sided. $\endgroup$
    – G_B
    Commented Apr 2, 2019 at 6:01
  • 4
    $\begingroup$ This seems to be a trivia question, not a puzzle... $\endgroup$
    – Chris
    Commented Apr 2, 2019 at 11:18
  • 3
    $\begingroup$ @user477343 consider Zeno's paradox :) -- However a polygon is made of a finite amount of edges by definition, so a circle (or any curved shape) still isn't a polygon. $\endgroup$
    – Quentin
    Commented Apr 2, 2019 at 12:39

3 Answers 3


I'm not entirely sure this is exactly a puzzle (but also not sure enough to suggest closing the question or anything). Anyway, I guess the reason is that

it's useful for them to be shapes of constant width (so that, e.g., they can go nicely into machines that accept coins for payment), and there's a nice simple construction for those that gives you a regular-polygon-ish shape with any odd number of sides; but nothing of that sort can possibly work for an even number of sides (because the diameter would have to be larger "between corners" than "in the middle of the side").

  • 1
    $\begingroup$ relevant link: Reuleaux polygons. $\endgroup$
    – G_B
    Commented Apr 2, 2019 at 6:05

Looking at this:


it is rarely true.

For example, Australia 2019:


  • 1
    $\begingroup$ @DrXorile; there's more recent later on $\endgroup$
    – JMP
    Commented Apr 1, 2019 at 18:19
  • $\begingroup$ I wonder how all these machines do in vending machines. Must make it tricky. $\endgroup$
    – Dr Xorile
    Commented Apr 2, 2019 at 19:49
  • $\begingroup$ @DrXorile The Australian 50c piece has been around in its current shape since 1969. I understand there are a few vending machines that don't accept it, but I can't recall ever encountering one, and by my understanding the issue is with its size rather than the shape. It's about 32 mm across, much larger than any other Australian coin, so distinguishing it from others isn't a challenge but everything has to be larger to accommodate it. $\endgroup$
    – G_B
    Commented Apr 3, 2019 at 1:44

Note that

The puzzle was posted on April 1st, a.k.a. April's Fools day

It turns out that

The post contains several falsehoods.

For example,

"An example" refers to an image with three coins. It is not in fact universally true that all coins are odd-sided (thanks JonMark Perry).


This question is not in fact a trivia question, but an Aprils Fools puzzle.

The answer is obviously

That I'm reading way too much into this. Possible ideas: "odd" refers to "odd one out". From the three coins, the larger 50p is no longer in production. The 20p is of a different denomination. The orientation in the picture may not be random. But somehow it should all link back to the coins being (roughly) polygonal...

  • $\begingroup$ Rot13: creuncf pbafvqre gung bqq pna zrna crephyvne be hahfhny. $\endgroup$ Commented Apr 2, 2019 at 15:59
  • $\begingroup$ Love it. I wish I were that clever... $\endgroup$
    – Dr Xorile
    Commented Apr 2, 2019 at 19:48
  • $\begingroup$ @DrXorile Aww man! I really thought there was a "Modern Polygonal Coin™" puzzle in here... $\endgroup$
    – Sanchises
    Commented Apr 2, 2019 at 19:52

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