Scotch Whisky must be matured in oak casks. However these casks can come from a variety of sources. Most whisky is matured in ex-bourbon casks from America. This is due to rules governing bourbon production whereby the casks can only be used once.
After this most are sent to Scotland for use in the whisky industry. The barrels can be used multiple times but the more often they are used then there is less impact on the flavour of the finished product. This is where the expression first fill, second fill etc comes from.
Some whiskies are matured in ex-Sherry casks which help to impart another dimension to the flavour. These help to add a richer, spicier taste to the whiskies. There are many variants of sherry, each of which add their own unique flavour.
Less commonly whiskies may be finished in red wine, white wine, port, calvados and even tequila casks. Each imparts it own unique flavour compounds to the finished product.
It is important to note some terminology here. Whiskies that have spent their entire maturation in a certain cask are said to be ‘cask matured’ whereas if they have only been ‘cask finished’ they have perhaps only spent a few months or years in the new cask after being transferred from another. As a result cask matured products take much more flavour from the cask into the final liquid.
One of the first, if not the first, Distilleries to experiment with cask finishing was The Balvenie with its Doublewood expression which spends almost 12 years in Bourbon casks before being transferred into sherry casks for a few months to impart an extra flavour dimension.