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The kg is defined as being equal to the mass of the International Prototype of the Kilogram (IPK), a block of platinum-iridium alloy manufactured in 1889 and stored at the International Bureau of Weights and Measures in Sèvres, France.

It is the only SI unit that is defined by a physical object rather than a fundamental physical property that can be reproduced in laboratories.

Abbreviated "ct." and spelled with a "c" is a measure of weight used for gemstones. One carat is equal to 1/5 of a gram (200 milligrams). Stones are measured to the nearest hundredth of a carat. A hundreth of a carat is also called a point. Thus a .10 carat stone can be called either 10 points, or 1/10 of a carat. Small stones like .05, and .10ct are most often referred to by point designations. Note that karat with a "K" is a measure of the purity of a gold alloy. A one carat round diamond of average proportions is approximately 6.5mm in diameter. Note that this relationship of weight and size is different for each family of stones. For example ruby and sapphire are both heavier than diamond (technically, they have a higher specific gravity, so a 1 carat ruby or sapphire is smaller in size than a on carat diamond. See Weights and Measures of Gold, Silver and Precious Gems for more information.