Gold is used extensively in coinage of course, as well as industrial, decorative, dental and gold jewellery.
How it is used will usually determine the carat or amount of gold in proportion to other metals. The higher the carat the purer the gold. Gold is usually alloyed with other metals to increase its hardness and durability. The amount of hardness required is determined by the use for the gold. Jewelry, for example, gets a lot more handling than coins or bars and so is alloyed with harder metals such as silver, copper and sometimes nickel.
The composition of alloys used in jewelry in most countries is denoted in the 'carat system' (spelt Karat in the USA and on the Continent). Also for hallmarking purposes gold is expressed in parts per thousand. Hence 999 and 990 parts per thousand (the other parts being another metal such as silver or copper for example).
Here is a list of the carats and proportions of gold to alloys:
24 carat is pure gold with nothing added. This is the purest gold available. Also has a fineness of 1000. Sometimes this is expressed as 999 being 999 parts per 1000. This is because it is very difficult to get pure gold with absolutely no impurities and possibly is for legal reasons also. This applies to gold coins a lot. Canadian Maples, for example, are listed as 99.999% pure gold.
22 carat is 916.6 fine
18 carat is 750 fine
14 carat is 585 fine
9 carat is 375 fine
There are other hallmark standards available as well as the above but these are the most common. This tells you how much gold there is in a gold piece. 14 carat, for example, is 585 parts gold to 415 parts other metal.
The alloys used vary with the purpose to which the gold is put. For jewellery and dental work usually the combination is gold, silver and copper. Sometimes with the addition of zinc or nickel. For dental alloys, palladium and platinum are usually added as they are particularly stronger and hard wearing.
18 carat gold is more popular for gold jewelry with a 75 percent gold and 25 percent other metals ratio, usually silver or copper or a mixture of both.
The 14 carat standard is used more extensively in industry and for such things as pen nibs, circuit boards etc. It is also used in such jewelry as bracelets where more durability is required, due to more use.
There is also a 10 carat, containing 41.7 percent gold and known therefore as 417. this is really just a cheaper version of the 14 carat and used for cheaper jewelry. One should check the carat of the piece one is buying as it may actually be 10 carat or even 9 carat and not 14.