All i could find on single malt is the following
Single malt ... contrary to common perception ... refers to Scotch whiskey made from 100% malted barley from a Single Distillery ... and not to the number of malts used.
A scotch called double malt (not an official nomenclature) ... is extremely uncommon ... and probably is refering to a vatted or blended malt scotch whiskey which combines malt whiskeys from just two distilleries. It would be officially known as a blended scotch, but that wouldn't give proper credit to its more exclusive pedigree. I have seen these sometimes called "Double Single" malt scotch whiskey.
Blended scotch is made from both malted barley and grain whiskeys and is usually sourced from many distilleries.
The reason single malts are considered superior nowadays is the use of only malt whiskeys and the more intense taste. They are also more expensive.
Blended scotch typically uses only 10-50% malt whiskey mixed with less expensive and less intensely flavored grain distillates. This was originally done because pure malt was considered too strongly flavored. Far more blended scotch is sold because of its softer flavor and lower price. Although you can have a blended scotch also of extremely high quality like Johnny Walker Blue.
All of this is not to be confused with "Double Wood" which refers to the use of multiple types of wood casks used in the aging process. Balvanie Double Wood for example uses casks made from American Oak previously used to age Bourbon and European Oak previously used to age sherry. Again the term is confusing since all their casks are from the single wood oak ... just from different sources and having different flavoring properties given their different prior usage.
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