A non-decimal currency is a currency that has sub-units that are a non-decimal fraction of the main unit, i.e. the number of sub-units in a main unit is not a power of 10. Historically, most currencies were non-decimal, though today virtually all are now decimal. Contemporary situation Today, only two countries have non-decimal currencies: Mauritania, where 1 ouguiya = 5 khoums, and Madagascar, where 1 ariary = 5 iraimbilanja. However, these are only theoretically non-decimal, as in both cases the value of each sub-unit is too small to be of any practical use and coins of sub-unit denominations are no longer used. The official currency of the Sovereign Military Order of Malta, which retains its claims of sovereignty under international law and has been granted permanent observer status at the United Nations, is the Maltese scudo, which is subdivided into 12 tarì, each of 20 grani with 6 piccioli to the grano. All other contemporary currencies are either decimal or have no sub-units at all, either because they have been abolished or because they have lost all practical value and are no longer used. Historic non-decimal currencies Historically, a variety of non-decimal systems have been used. For example, A vigesimal system (base 20) was in use within ancient Mesoamerica. A sexagesimal system (base 60) was in wide use in ancient Mesopotamia, as this system was used in measurements of time, geometry, currency, and other fields. Decimal currencies also have disadvantages. The principal advantage of most non-decimal currencies is that they are more easily divided, particularly by numbers such as 3 and 8, than decimal currencies, due to being based upon conversion values that have a large number of factors. A currency with a 100:1 ratio is divisible neither into 3 nor into 8. For example, one-third of an Austrian Gulden (of 60 Kreuzer) was 20 Kreuzer while a third of a dollar is 33. cents. This divisibility is useful when trading and when sharing out sums of money. For these reasons, many states chose in the past to adopt non-decimal currencies based on divisions into sub-units such as 12 or 20, sometimes with more than one tier of sub-units. There is a second, more fortuitous, way in which non-decimal currencies emerged. Often multiple currencies would circulate concurrently in an economy, with non-decimal exchange rates between them. For example, a group related currencies called Reichsthaler, rixdollar, riksdaler, rijksdaalder, and rigsdaler were