Scotch whisky is divided into five different categories:
Single Malt Scotch Whisky
Scotch Whisky produced from only water and malted barley at a single distillery by batch distillation in pot stills.
Single Grain Scotch Whisky
Scotch Whisky distilled at a single distillery but, in addition to water and malted barley, may involve whole grains of other malted or unmalted cereals. “Single Grain” does not mean that only a single type of grain was used to produce the whisky—rather, the adjective “single” refers only to the use of a single distillery (and making a “Single Grain” requires using a mixture of grains, as barley is a type of grain and some malted barley must be used in all Scotch whisky).
Excluded from the definition of “Single Grain Scotch Whisky” is any spirit that qualifies as a Single Malt Scotch Whisky or as a Blended Scotch Whisky. The latter exclusion is to ensure that a Blended Scotch Whisky produced from Single Malt(s) and Single Grain(s) distilled at the same distillery does not also qualify as Single Grain Scotch Whisky.
Blended malt Scotch Whisky (formerly called “vatted malt” or “pure malt”)
a blend of two or more Single Malt Scotch Whiskies from different distilleries.
Blended Grain Scotch Whisky
A blend of two or more Single Grain Scotch Whiskies from different distilleries.
Blended Scotch Whisky
A blend of one or more Single Malt Scotch Whiskies with one or more Single Grain Scotch Whiskies
All Scotch whisky must be aged in oak barrels for at least three years. Any age statement written on a bottle of Scotch whisky, in the form of a number, must reflect the age of the youngest whisky used to produce that product. A whisky with an age statement is known as guaranteed age whisky.