A blended grain scotch whisky is the blend of two or more single grain scotch whiskies from different distilleries.
Grain whisky is usually made from 10-20% malted barley and then other unmalted cereals such as maize or wheat. So the ingredients are different from Malt Whisky where it is 100% malted barley. The still used to make grain whisky, a Coffey still named after its inventor Aeneas Coffey, is also different and can be run continuously rather than in batches when making a single malt. It has two tall columns - a rectifier and an analyser and it produces Scotch grain spirit at about 94% alcohol by volume. The distilled grain spirit is lighter in character and aroma than most malt whiskies and therefore requires rather less time to mature. The bulk of matured grain whisky is used for blending. Distillers discovered that if you leave a grain whisky maturing for longer than 20 years the flavours are extremely delicate and sweet.