coin

A coin is usually a piece of hard material, usually metal or a metallic material, usually in the shape of a disc, and most often issued by a government. Coins are used as a form of money in transactions of various kinds, from the everyday circulation coins to the storage of vast amounts of bullion coins. In the present day, coins and banknotes make up the cash forms of all modern money systems. Coins made for circulation (general monetized use) are usually used for lower-valued units, and banknotes for the higher values; also, in most money systems, the highest value coin is worth less than the lowest-value note. The face value of circulation coins is usually higher than the gross value of the metal used in making them, but this is no longer generally the case with historical circulation coins made of precious metals. For example, the historical Eagle (U.S. coin) contained .48375 troy ounce of gold and has a face value of only ten U.S. dollars, but the market value of the coin, due to its metal content, is now many times the face amount.

Exceptions to the rule of coin face-value being higher than content value, also occur for some non-monetized "bullion coins" made of silver or gold (and, rarely, other metals, such as platinum or palladium), intended for collectors or investors in precious metals. For examples of modern gold collector/investor coins, the United States mints the American Gold Eagle, Canada mints the Canadian Gold Maple Leaf, and South Africa mints the Krugerrand. The American Gold Eagle has a face value of US$50, and the Canadian Gold Maple Leaf coins also have nominal (purely symbolic) face values (e.g., C$50 for 1 oz.); but the Krugerrand does not.

Historically, a great number of coinage metals (including alloys) and other materials have been used practically, impractically (i.e., rarely), artistically, and experimentally in the production of coins for circulation, collection, and metal investment, where bullion coins often serve as more convenient stores of assured metal quantity and purity than other bullion.

Coins have long been linked to the concept of money, as reflected by the fact that in other languages the words "coin" and "currency" are synonymous. Fictional currencies may also bear the name coin (as such, an item may be said to be worth 123 coin or 123 coins).
Features of modern coinage

A bronze coin of the Chinese Han Dynasty—circa 1st century BC. Some modern Japanese coins still have the characteristic hole in the coin.
An ancient Greek coin, struck under Roman rule, circa 268 AD.
British fifty pence coin

Gold sovereigns and a Krugerrand
Republic of China Ten Cash
A set of US coins.The milled, or reeded, edges still found on many coins (always those that were once made of gold or silver, even if not so now) were originally designed to show that none of the valuable metal had been shaved off the coin. Prior to the use of milled edges, circulating coins commonly suffered from "shaving", by which unscrupulous persons would shave a small amount of precious metal from the edge. Unmilled British sterling silver coins were known to be shaved to almost half of their minted weight. This form of debasement in Tudor England led to the formulation of Gresham's Law. The monarch would have to periodically recall circulating coins, paying only bullion value of the silver, and re-mint them.

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. 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 British 20p coin, and all Japanese coins, are an exception.

The orientation of the obverse with respect to the reverse differs between countries. Some coins have coin orientation, where the coin must be flipped vertically to see the other side; other coins, such as British coins, have medallic orientation, where the coin must be flipped horizontally to see the other side.

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, and they are most common on coins with little or no legends such as the Victorian bun penny.

Coins that are not round (British 50 pence for example) usually have an odd number of sides , with the edges rounded off. This is so that the coin has a constant diameter, and will therefore be recognised by vending machines whichever way it is inserted[12]. The triangular coin (produced to commemorate the 2007/2008 Tutankhamun exhibition at the The O2 Arena) was commissioned by the Isle of Man, became legal tender on 6 December 2007[13] and has a value of 25p (a crown). The triangular coin issued to commemorate the return of Tutankhamun treasures to London was not the first coin with a triangular shape. Some triangular coins produced earlier include : Cabinda coin, Bermuda coin, 2 Dollar Cook Islands 1992 triangular coin, Uganda Millennium Coin and Polish Sterling-Silver 10-Zloty Coin.

Coins are popularly used as a sort of two-sided die; in order to choose between two options with a random possibility, one choice will be labeled "heads" and the other "tails," and a coin will be flipped or "tossed" to see whether the heads or tails side comes up on top. See Bernoulli trial; a fair coin is defined to have the probability of heads (in the parlance of Bernoulli trials, a "success") of exactly 0.5. A widely publicized example of an asymmetrical coin which will not produce "fair" results in a flip, is the Belgian one euro coin . See also coin flipping. Coins are sometimes falsified to make one side weigh more, in order to simulate a fair type of coin which is actually not fair. Such a coin is said to be "weighted."

Some coins, called bracteates, are so thin they can only be struck on one side.

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.

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

The Royal Canadian Mint is now able to produce holographic-effect gold and silver coinage.

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

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