The Dewar's 25 Years Old The Signature Double Aged is a premium blended Scotch. Specially selected casks were blended together and then filled into oak casks for an additional period of maturation, a process pioneered by Dewar's, known as double-ageing, to add more depth and enriched smoothness.During this extra period of maturation, the disparate characters of the malts and grains are allowed to interact with one another and mellow further. In an extra step unique to the new 25-year-old expression, the whisky was then filled into a set of freshly disgorged Royal Brackla casks for an extra period of finishing.