What is a single malt whisky? In short: a Single Malt whisky is a single distillate from one single malt whisky distillery. Up to this stage there is certainly no blending.
In the longer version of the term Single Malt Whisky is understood to mean that the whisky was made using only barley as the raw material, that it was distilled in a copper pot still and that it was aged in oak casks for at least three years at a single distillery. Once we add the ageing process into the mix we get to the territory of blending.
The blending usually occurs after ageing. The master distiller blends together different types of casks to match the single malt expression he is making. The Glenlivet 'Nadurra' 16 year old single malt uses ex-bourbon cask exclusively and therefore requires no blending. Glenfiddich 15 year old single malt is a combination of ex-sherry casks and ex bourbon casks.
TO BE VERY CLEAR
It remains a single malt whisky even though we have blended from different cask. All the whisky still came from a single distillery. It is not a blended malt whisky which will contain malts from many distilleries. It is not a blended scotch whisky that contain many single malts and also contains grain whisky.
Balvenie Double Wood is a single malt whisky. It is blended using two types of oak casks. The first six years of this 12 year old single malt whisky is spent resting in ex-bourbon casks before being transferred into ex-sherry casks for the final six years 6 years - hence 'Double Wood'. Many people asked me about this "double malt". LOL!
Most other single malt whisky distilleries age their single malt whiskies in one type of cask for the entire ageing period. It is only at the end of the ageing period that they blend together a combination of ex-sherry and ex-bourbon casks in proportion to the flavour they and style of their single malt.
The Glenlivet 12 year old is mostly ex-bourbon barrel, The Glenlivet 15 year is a higher combination of ex sherry cask versus ex-bourbon cask and includes malt that was aged in ex-cognac casks. The Glenlivet 18 years has an even higher ex-sherry cask content than the 12, 15 or 16 year old.
From the above you now also understand that an 18 year old whisky is not simply the same 12 year old whisky aged for another 6 years. It is a new expression that remains true to the entire range, but it is a different blend of casks. You will always know its a The Glenlivet no matter the age statement other than it being smoother.