In Scotland, malt whisky is made from 100% malted barley, in pot stills. This is a batch process--that is, the whisky is made one batch at a time. The still is filled, then run until the wash is spent, and then the process is repeated. Single malt is malt whisky from one distillery. Double malt is a bogus term, with no legitimate definition. Grain whisky is made in column stills. This is a continuous process--the stills can be fed and run continually for as long as desired. Any grain can be used-- corn is usual these days, because it's the cheapest. The purpose of grain whisky is to provide inexpensive bulk for blends, with malts providing the desired flavors. As such, it's not common for grain whisky to be bottled on its own, but it is done. I recall a review of one single grain bottling in WM that was very effusive, owing to the fact that, at the time it was made, barley was the cheapest grain available, and thus the product was more like a malt than most grains (which tend to taste bourbony), although it was made in a column still.