The karat number of gold refers to how pure the gold is. Outside the United States and Canada, the spelling 'carat' is used instead. 24 karat gold (24k) is absolutely pure gold. Karat is also used as a measurement for the purity of platinum, although this is much less commonly encountered.
The spelling shift from 'c' to 'k' is because carat is also a measurement of gem mass, with one carat being equal to 200 milligrams (mg). This is also known as the metric carat.
Karat purity is measured by mass. 18 karat gold is 75% pure gold by mass.
Divisions beyond one part in 24 are also possible. In the old British measurement system, four grains made a carat, and four quarts made a grain.
Modern decimal systems use the millesimal fineness system instead, either to supplement the karat system or to supersede it. In the millesimal fineness system, the purity of gold is measured by parts per thousand of pure metal, as measured by mass.
Absolutely pure gold is too soft for most non-electronic purposes. 14k, 18k, 20k, and 22k are some of the most common jewelry alloys, being strong enough to hold their shape while still retaining most of the desirable attributes of pure gold.
24k gold is always pure yellow in colour. Alternate colours for gold, such as orange, rose, green, or white, automatically mean that it has been alloyed with some other metal. Most common for this purpose are silver, copper, nickel, or palladium, depending upon the exact effect desired. Manganese, iron, aluminum, iridium, and even other metals are sometimes used to achieve unusual effects or to be used for specific types of jewelry. Nickel alloys are used for hardness, while palladium alloys are soft, and thus ideal for gem settings.
Jeweler's marks indicating purity vary from country to country. Two of the most common are 14k, as used in this article, and representing the number as a ratio (14/24), with or without k.
Another variant on what might seem a ratio is the Indian subcontinent custom of indicating skin purity (referring to the top layer of the jewelry) and melting purity (referring to total composition).
Because of its high value, most countries allow 1% or less tolerance in gold percentage deviations. This is usually regulated not only by industry regulation but also by law. The same legislation usually tightly restricts what may be called gold, gold-filled, gold-plated, gold-overlay, gold-electroplated, and so on.