Karat is the weight measurement equal to one twenty-fourth part of the metallic element gold in an alloy. The Byzantine coin, the solidus, was divided in 24 Keratia. From this our present day purity system, the karat standard, has been derived. We express the amount of gold present in an alloy into parts of 24. This means that 18 karat gold contains a 18/24th part of pure gold. Which can also be expressed as the number '750' which stands for 750/1000 (750 parts out of thousand, or 75%). Pure gold is denoted as being 24 karats.