Most coins are circular because of ease of production and their shape does not wear holes in clothes pockets. The most common non-circular coin we are familiar with is the Australian 50 cent piece which has 12 sides and called a dodecagon. We were presented by one of our members with a beautiful collection of silver and gold 50 cent coins (see photo below), most of which none of us have ever seem; what a treat!
We were also presented with a number of other non-circular coins including a 7-sided 50 cent piece (heptagon), a 12-sided three pence from the UK and a Hong Kong 12 point scalloped 20 cent piece.
Most of our modern circulating coins are round because a round shape is less likely to wear down unevenly since no one part of it sticks out further than the rest from any angle.
Also, in the days when coin value was assessed by metal type and weight, it was easy to cut off the corners but more difficult to do that with round coins. Milling of the coin edges also limited this practice especially when coins were made of precious metals like gold and silver. Then in the modern world there were vending machines which need coins to roll.
Coin of the Month for September will be “let’s talk about the Australian penny and its dots”.