Cask maturation

The rules differ by country. Scotch Whisky must be matured only in Scotland, only in oak casks of not more than 700 litres, for not less than three years. Irish whiskey must spend at least three years on the island of Ireland in wooden casks, such as oak, of not more than 700 litres; casks that previously held Madeira, sherry, port or bourbon are used. US bourbon and rye must be stored in charred new oak barrels, entering at no more than 125 US proof (62.5% abv); a "straight" whisky has at least two years in such barrels. Canadian whisky is aged in small wood for at least three years, and Japanese whisky under the 2021 industry standard for at least three years in wooden casks of not more than 700 litres.

Age statements on Scotch Whisky must be in years and refer to the youngest whisky in the bottle.

During maturation the spirit takes colour and flavour from the cask and interacts with it; the Irish technical file notes that seasoned casks give a balanced character without excessive wood extracts and tannins. A review of the research by J. R. Mosedale (1995) found that the chemical and anatomical properties of oak that may influence flavour vary both in new wood and in used casks, and concluded that choosing wood or casks for their effect on flavour is feasible even though the processes are not fully understood.