As well as the carat system, there were other historical standards for measuring the purity of gold. For instance, from the 10th century until 1927, Russia used the Zolotnik System as did Russian influenced countries. This system was based on the number 96, originating from a gold coin equal in weight to the Roman solidus, and purity was stamped as 94, 92, 82, 72 and 56.  There were numerous other systems which have disappeared when countries merged, split and were re-invented. They do help identify the age of a piece of antique jewellery.

Today, the main system used to measure the purity of gold is the millesimal system. The millesimal system expresses the purity of gold (and other metals) out of 1000 (really 999). So 9ct gold is .375, 15ct gold is 0.625 and 18ct gold is 0.750. This system is used by most European countries. The UK used to use only the carat system but recognized both systems from 1854 and changed completely to the millesimal system in 1975. Australia started to use the millesimal system in 2008. Canada recognized the millesimal system in 1995.

I should also clarify that the purity of gold is not the same as the weight of gold. The weight of gold is measured in troy ounces (1 troy ounce = 31.1034768 grams). Troy ounces take their name from the French city of Troyes in the Middle Ages, although are believed to have originated from a weights systems developed in Roman times. A troy ounce weighs a bit more than a normal ounce. You may see some jewellery weighed using this system.