A cask is the wooden vessel that whisky is stored in while it matures and it plays a critical role in developing flavours. Casks can come in all different sizes and wood types, but the most common type used for whisky is a 200L ex-bourbon barrel that was originally used to mature American Bourbon. That said, whisky is regularly matured in other types of casks including sherry, wine and tequila, all of which bring their own flavours to the spirit.