Features of modern coins

Circulating coins commonly suffered from "shaving" or "clipping", by which persons would cut off small amounts of precious metal from their edges to form new coins. Unmilled British sterling silver coins were sometimes reduced to almost half their minted weight. This form of debasement in Tudor England was commented on by Sir Thomas Gresham, whose name was later attached to Gresham's Law. 

The monarch would have to periodically recall circulating coins, paying only bullion value of the silver, and reminting them. This, also known as recoinage, is a long and difficult process that was done only occasionally. Many coins have milled or reeded edges, originally designed to make it easier to detect clipping.

Traditionally, the side of a coin carrying a bust of a monarch or other authority, or a national emblem, is called the obverse, or colloquially, heads; see also List of people on coins. The other side is called the reverse, or colloquially, tails. However, the rule is violated in some cases. Another rule is that the side carrying the year of minting is the obverse, although some Chinese coins, most Canadian coins, the pre-2008 British 20p coin, and all Japanese coins, are exceptions.

In cases where a correctly oriented coin is flipped vertically to show the other side correctly oriented, the coin is said to have coin orientation. In cases where a coin is flipped horizontally to show the other side, it is said to have medallic orientation. While coins of the United States Dollar are coin orientated, those of the Euro and British Pound are medallic.

Bi-metallic coins are sometimes used for higher values and for commemorative purposes. In the 1990s, France used a tri-metallic coin. Common circulating examples include the €1, €2, British £2 and Canadian $2.

The exergue is the space on a coin beneath the main design, often used to show the coin's date, although it is sometimes left blank or containing a mint mark, privy mark, or some other decorative or informative design feature. Many coins do not have an exergue at all, especially those with few or no legends, such as the Victorian bun penny.

Not all coins are round. The Australian 50 cent coin, for example, has twelve flat sides. A twist on it is wavy edges, found in the two dollar and the twenty cent coins of Hong Kong and the 10 cent coins of Bahamas. Some coins have also been issued in the shape of a square, such as the 15 cent coin of the Bahamas. During the 1970s, Swazi coins were minted in several shapes, including squares, polygons, and wavy edged circles with 8 and 12 waves.

Some other coins, like the British Fifty pence coin and the Canadian Loonie, have an odd number of sides, with the edges rounded off. This way the coin has a constant diameter, recognisable by vending machines whichever direction it is inserted.

A triangular coin with a face value of five pounds (produced to commemorate the 2007/2008 Tutankhamun exhibition at The O2 Arena) was commissioned by the Isle of Man, it became legal tender on 6 December 2007. Other triangular coins issued earlier include: Cabinda coin, Bermuda coin, 2 Dollar Cook Islands 1992 triangular coin, Uganda Millennium Coin and Polish Sterling-Silver 10-Zloty Coin.

Guitar-shaped coins were once issued in Somalia. Poland once issued a fan-shaped 10 złoty coin and the 2002 $10 coin from Nauru, was Europe-shaped.

Some mediaeval coins, called bracteates, were so thin they were struck on only one side.

The Royal Canadian Mint is now able to produce holographic-effect gold and silver coinage. However this procedure is not limited to only bullion or commemorative coinage. The 500 yen coin from Japan, was subject to a massive amount of counterfeiting. The Japanese government in response produced a circulatory coin with a holographic image.

The Royal Canadian Mint has also released several coins that are coloured, the first of which was in commemoration of Remembrance Day. The subject was a coloured poppy on the reverse of a 25 cent piece.

For a list of many pure metallic elements and their alloys which have been used in actual circulation coins and for trial experiments, see coinage metals.