Both the specification 18 carat and the designation 750 gold indicate the total proportion of pure gold within a gold alloy, i.e. the fusion of different metals. The carat specification for gold is a unit of measurement for the proportion by weight that the pure gold occupies in the overall mixture of the alloy, also known as the fineness. The fineness is given in 1/24 parts. The largest possible weight percentage of pure gold within a gold alloy would be 24/24 weight percentages, i.e. 24 carats. However, the fineness of gold can also be stated in percent by weight. 18 carats then correspond to exactly 75% gold in the total mass of the alloy at the highest possible 24 carat fineness. The designation 750 gold, based on the 75% gold content, therefore describes the same purity of the alloy and can be used synonymously..