Scotch whisky (; ; ) often simply called whisky or Scotch) is malt whisky or grain whisky (or a blend of the two), made in Scotland. All Scotch whisky was originally made from malted barley. Commercial distilleries began introducing whisky made from wheat and rye in the late 18th century. , there were 141 whisky distilleries operating in Scotland. All Scotch whisky must be aged immediately after distillation in oak barrels for at least three years. Any age statement on a bottle of Scotch whisky, expressed in numerical form, must reflect the age of the youngest whisky used to create that product. A whisky with an age statement is known as guaranteed-age whisky. A whisky without an age statement is known as a no age statement (NAS) whisky, the only guarantee being that all whisky contained in that bottle is at least three years old. The minimum bottling strength according to existing regulations is 40% alcohol by volume. Scotch whisky is divided into five distinct categories: single malt Scotch whisky, single grain Scotch whisky, blended malt Scotch whisky (formerly called "vatted malt" or "pure malt"), blended grain Scotch whisky, and blended Scotch whisky. The first known written mention of Scotch whisky is in the Exchequer Rolls of Scotland of 1494. Many Scotch whisky drinkers refer to a unit for drinking as a dram. According to the Scotch Whisky Association, the word whisky comes from the Gaelic uisge beatha or usquebaugh, which means "water of life". History The earliest record of distillation in Scotland is in the Exchequer Rolls of Scotland for 1494. The Exchequer Rolls' record crown income and expenditure and the quote records eight bolls of malt given to Friar John Cor to make aqua vitae over the previous year. The term is Latin for "water of life" and was the general term for distilled spirits. This would be enough for 1,500 bottles, which suggests that distillation was well-established by the late 15th century. The first known reference to a still for making "aquavite" in Scotland appears in the Aberdeen council registers, in a case heard in 1505 by the town's bailies concerning the inheritance of goods belonging to a chaplain called Sir Andrew Gray, who died in 1504. Among his goods was recorded (in Middle Scots) "ane stellatour for aquavite and ros wattir". It is probable that whisky was introduced to Scotland from Ireland as there is evidence