Most coins are circular, but 20p and 50p coins are different. Instead of being circular, they are curved heptagons (seven-sided shapes).
All coins need to be able to roll, so that they can be used in vending machines. 20p and 50p coins can roll just as well as circular coins, even though they are not circular.
This is because they are Reuleaux curves. Reuleaux curves have curved corners and slightly curved edges, and always have an odd number of sides. This means that they are always the same width all of the way around. Circles are also the same width all the way round. This is why circles and Reuleaux curves roll.
The constant width of these coins also means that they can be identified using coin detectors in vending machines. These detectors identify coins and check they are genuine by measuring their weight, width and/or magnetism. It is, therefore, important that a measurement of width from any angle will give the same width.