As whisky matures, it continually interacts with the wood in which it lies. Over time, this interaction subtly changes the whisky’s flavour profile. Due to a steady supply and low cost, casks sourced from the United States, which previously held bourbon, are most commonly used to mature Scotch Whisky.
More exotic casks, such as ex-sherry or rum, can be dramatically more expensive than bourbon and therefore they aren’t always an option to age whisky for the full maturation period. That said, these casks can still be used to influence the whisky, thanks to a process called re-racking.